Color image forming apparatus

ABSTRACT

A color image forming apparatus of a so-called tandem type which has image forming units in correspondence with colors is provided. In the color image forming apparatus, a color discrepancy amount storage unit stores information of a color discrepancy amount of each of the image forming units, which is measured in advance. A first color discrepancy correcting unit performs color discrepancy correction in a pixel unit by performing coordinate conversion of bitmap data to be printed based on the information of the color discrepancy amount stored in the color discrepancy amount storage unit. A second color discrepancy correcting unit performs color discrepancy correction in less than a pixel unit by performing tone correction of the bitmap data corrected by the first color discrepancy correcting unit based on the information of the color discrepancy amount stored in the color discrepancy amount storage unit.

FIELD OF THE INVENTION

The present invention relates to a color image forming apparatus and,more particularly, to a color image forming apparatus of anelectrophotographic method.

BACKGROUND OF THE INVENTION

A color image forming apparatus of an electrophotographic method is wellknown. In this method, one photosensitive member undergoes developmentsof respective colors using a plurality of developers, color images aresuperposed and formed on a single transfer material by repeatingexposure, development, and transfer processes a plurality of number oftimes, and these color images are fixed to obtain a full-color image.

However, this method must repeat three (four if black is used) imageforming processes to obtain a print image on one sheet, resulting in along image forming time.

As a system that can cope with this drawback, a so-called tandem systemwhich superposes visual images obtained for respective colors using aplurality of photosensitive members to obtain a full-color print via asingle paper feed operation is known. According to this tandem system,the throughput can be greatly improved. On the other hand, a colordiscrepancy program has occurred due to misalignments of respectivecolors on a transfer material resulting from errors of the positionalprecisions and diameters of photosensitive members, and the positionalprecision errors of optical systems, and it is difficult to obtain ahigh-quality full-color image.

Various measures against this color discrepancy have been proposed. Forexample, Japanese Patent Application Laid-Open No. 64-40956 (parentreference 1) discloses a technique which forms a test toner image on atransfer material or a transfer belt which forms a transfer unit,detects the formed image, and corrects the optical path of each opticalsystem or corrects the image write start position of each color based onthe detection result.

Japanese Patent Application Laid-Open No. 8-85237 (patent reference 2)discloses the following technique. The output coordinates of image dataof respective colors are converted into those free from any registrationerrors. After that, based on the converted image data of respectivecolors, the positions of modulated light beams are corrected by anamount less than the minimum dot unit of a color signal.

However, the method disclosed in patent reference 1 poses, e.g., thefollowing problems.

First, in order to correct the optical path of the optical system, acorrection optical system including a light source and f-θ lens, amirror in the optical path, and the like must be mechanically moved toadjust the position of the test toner image. For this purpose,high-precision movable members are required, resulting in high cost.Furthermore, since it takes much time until correction is completed, itis impossible to frequently perform correction. However, an optical pathlength difference may change along with an elapse of time due totemperature rise of mechanical components. In such case, it becomesdifficult to prevent color discrepancy by correcting the optical path ofthe optical system. Second, in order to correct the image write startposition, it is possible to conduct misalignment correction of the leftend and upper left portion but it is impossible to correct the tilt ofthe optical system and to correct any magnification errors due to theoptical path length difference.

The method disclosed in patent reference 2 poses, e.g., a problem of alarge calculation volume since color discrepancy correction amounts mustbe calculated for all pixels. FIGS. 1A and 1B show an example. An imageshown in FIG. 1A has a constant density value. In order to obtain animage shown in FIG. 1B by applying arbitrary color discrepancycorrection to this input image, the density values corresponding to allpixels must be calculated. For this reason, the calculation volumebecomes large, and the arrangement of a processing system becomescomplicated.

SUMMARY OF THE INVENTION

In view of the above problems in the conventional art, the presentinvention has an object to provide a color image forming apparatus whichcan obtain a high-quality color image by reducing color discrepancywithout large increase in cost.

According to one aspect of the present invention, there is provided acolor image forming apparatus of a so-called tandem type which comprisesimage forming units in correspondence with colors. This color imageforming apparatus comprises a color discrepancy amount storage unitconfigured to store information of a color discrepancy amount of each ofthe image forming units, which is measured in advance, a first colordiscrepancy correcting unit configured to perform color discrepancycorrection in a pixel unit by performing coordinate conversion of bitmapdata to be printed based on the information of the color discrepancyamount stored in the color discrepancy amount storage unit, and a secondcolor discrepancy correcting unit configured to perform colordiscrepancy correction in less than a pixel unit by performing tonecorrection of the bitmap data corrected by the first color discrepancycorrecting unit based on the information of the color discrepancy amountstored in the color discrepancy amount storage unit.

The above and other objects and features of the present invention willappear more fully hereinafter from a consideration of the followingdescription taken in connection with the accompanying drawings whereinone example is illustrated by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the description, serve to explain the principles of theinvention.

FIGS. 1A and 1B are views for explaining conventional color discrepancycorrection processing;

FIG. 2 is a schematic sectional view showing the arrangement of a colorimage forming apparatus according to the first embodiment of the presentinvention;

FIG. 3 is a view for explaining a misalignment of a main scan linescanned on a photosensitive drum;

FIG. 4 is a control block diagram associated with color discrepancycorrection processing according to the first embodiment of the presentinvention;

FIG. 5 is a table showing an example of information stored in a colordiscrepancy amount storage unit in the first embodiment of the presentinvention;

FIGS. 6A to 6C are views for explaining correction of discrepancyamounts in the pixel unit by a coordinate converter in the firstembodiment of the present invention;

FIGS. 7A to 7G are views for explaining discrepancy correction in lessthan the pixel unit by a tone corrector in the first embodiment of thepresent invention;

FIG. 8 is a block diagram showing the detailed arrangement of a colordiscrepancy correcting unit in the first embodiment of the presentinvention;

FIGS. 9A to 9G are views for explaining discrepancy correction in lessthan the pixel unit by a tone corrector in the second embodiment of thepresent invention;

FIG. 10 is a view for explaining a misalignment of a main scan linescanned on each photosensitive drum of a color image forming apparatusaccording to the third embodiment of the present invention;

FIG. 11 is a control block diagram associated with color discrepancycorrection processing according to the third embodiment of the presentinvention;

FIG. 12 is a table showing an example of data stored in a colordiscrepancy amount storage unit according to the third embodiment of thepresent invention;

FIG. 13 is a block diagram showing the arrangement of a colordiscrepancy correcting unit according to the third embodiment of thepresent invention;

FIG. 14 is a conceptual view for explaining the operation contents whena coordinate converter according to the third embodiment of the presentinvention corrects a discrepancy amount of the integer part of a colordiscrepancy correction amount Δy, i.e., color discrepancy for each line;

FIG. 15 is a conceptual view for explaining the operation contents whena tone corrector according to the third embodiment of the presentinvention performs color discrepancy correction in less than a dot unit,i.e., it corrects a discrepancy amount of the decimal part of the colordiscrepancy correction amount Δy;

FIG. 16 is a block diagram showing an example of a controller shown inFIG. 11 which is configured by a CPU and memories;

FIGS. 17 and 18 are flowcharts for explaining image forming processingto be executed by the CPU of the controller according to the thirdembodiment of the present invention;

FIG. 19 is a control block diagram associated with color discrepancycorrection processing according to the fourth embodiment of the presentinvention;

FIG. 20 is a block diagram showing the arrangement of a colordiscrepancy correcting unit according to the fourth embodiment of thepresent invention;

FIGS. 21A to 21C are conceptual views for explaining the operationcontents when a coordinate converter according to the fourth embodimentof the present invention corrects a discrepancy amount of the integerpart of a color discrepancy correction amount Δy (color discrepancy foreach line);

FIGS. 22A to 22F are conceptual views for explaining the operationcontents when a tone corrector according to the fourth embodiment of thepresent invention performs color discrepancy correction in less than adot unit, i.e., it corrects a discrepancy amount of the decimal part ofthe color discrepancy correction amount Δy;

FIGS. 23 and 24 are flowcharts for explaining image forming processingto be executed by a CPU of a controller according to the fourthembodiment of the present invention;

FIG. 25 is a control block diagram associated with color discrepancycorrection processing according to the fifth embodiment of the presentinvention;

FIG. 26 is a table showing an example of data stored in a colordiscrepancy amount storage unit according to the fifth embodiment of thepresent invention;

FIG. 27 is a block diagram showing the arrangement of a colordiscrepancy correcting unit according to the fifth embodiment of thepresent invention;

FIGS. 28A to 28C are views illustrating the operation contents when acoordinate converter according to the fifth embodiment of the presentinvention corrects a discrepancy amount of the integer part of a colordiscrepancy correction amount Δy;

FIGS. 29A to 29F are views illustrating the operation contents of colordiscrepancy correction in less than the pixel unit performed by a tonecorrector according to the fifth embodiment of the present invention;

FIGS. 30A to 30H are views illustrating processing for assigning manybits to the number of bits of data output from a bitmap memory uponperforming tone correction according to the fifth embodiment of thepresent invention;

FIGS. 31A to 31D are views for explaining general image distortioncorrection;

FIGS. 32A to 32C are views for explaining occurrence of colorinconsistency resulting from the general image distortion correction;

FIG. 33A is a sectional view showing the internal structure of an imageforming apparatus according to the sixth embodiment of the presentinvention;

FIG. 33B is a view showing misalignment of an optical system in theimage forming apparatus according to the sixth embodiment of the presentinvention;

FIG. 34 is a block diagram showing the arrangement for positionmisalignment adjustment according to the sixth embodiment of the presentinvention;

FIGS. 35A to 35C are views for explaining image distortion correctionaccording to the sixth embodiment of the present invention;

FIG. 36 is a view for explaining details of adjustment of a scan startposition according to the sixth embodiment of the present invention;

FIG. 37 is a timing chart showing the relationship between a videosignal and horizontal sync signal according to the sixth embodiment ofthe present invention;

FIG. 38 is a block diagram showing the arrangement according to theseventh embodiment of the present invention;

FIG. 39 is a block diagram showing the arrangement according to theeighth embodiment of the present invention;

FIG. 40 is a block diagram showing the arrangement according to theninth embodiment of the present invention;

FIG. 41 is a timing chart showing the relationship between a videosignal and horizontal sync signal according to the ninth embodiment ofthe present invention; and

FIG. 42 is a block diagram showing the arrangement according to thetenth embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described indetail in accordance with the accompanying drawings. The presentinvention is not limited by the disclosure of the embodiments and allcombinations of the features described in the embodiments are not alwaysindispensable to solving means of the present invention.

First Embodiment

FIG. 2 is a schematic sectional view showing the arrangement of a colorimage forming apparatus according to this embodiment. A color imageforming apparatus 1 shown in FIG. 2 is a color laser beam printer of aso-called tandem system, which comprises, e.g., four photosensitivedrums. This color image forming apparatus 1 mounts a transfer materialcassette 53 in a lower portion of the right side surface of its mainbody. Transfer materials set in the transfer material cassette 53 arepicked up one by one by a paper feed roller 54, and each transfermaterial is fed to image forming units by guide roller pairs 55-a and55-b. A feeding belt 10 that feeds the transfer material is stretchedflat via a plurality of rotary rollers in the transfer material feedingdirection (from the right to the left in FIG. 2), and the transfermaterial is electrostatically attracted on the most upstream portion ofthe feeding belt 10.

The color image forming apparatus 1 has four image forming units(sometimes referred to as “printer engines” or “image station”) 50-C,50-Y, 50M, and 50-K, which are juxtaposed in turn from the upstream sidealong the feeding belt 10. The printer engine 50-C forms an image usingC (CYAN) toner. The printer engine 50-Y forms an image using Y (YELLOW)toner. The printer engine 50-M forms an image using M (magenta) toner.The printer engine 50-K forms an image using K (BLACK) toner. Theseprinter engines respectively comprise photosensitive drums 14-C, 14-M,14-Y, and 14-K as drum-shaped image carriers, which face the beltconveyor surface of the feeding belt 10. This is the basic arrangementof the so-called tandem system. Since the detailed arrangements of therespective printer engines are basically the same, the arrangement ofthe printer engine 50-C will be described as a representative, and adescription of the arrangements of other printer engines will beomitted.

The printer engine 50-C comprises an exposure unit 51-C, developing unit52-C, and transfer member 57-C in addition to the photosensitive drum14-C. The exposure unit 51-C includes a laser scanner, and thedeveloping unit 52-C includes C (CYAN) toner, a charger, and adeveloper. A predetermined gap is formed between the charger anddeveloper in the housing of the developing unit 52-C, and thecircumferential surface of the photosensitive drum 14-C is uniformlycharged by a predetermined charge from the exposure unit 51-C via thisgap. The exposure unit 51-C exposes the circumferential surface of thephotosensitive drum 14-C in accordance with image information to form anelectrostatic latent image, and the developer transfers toner to alow-potential part of the electrostatic latent image to develop a tonerimage.

The transfer member 57-C is arranged on the other side of the conveyorsurface of the feeding belt 10. The toner image formed (developed) onthe circumferential surface of the photosensitive drum 14-C is attractedby a charge, which is generated on the fed transfer material by atransfer electric field formed by the transfer member 57-C, and istransferred onto the surface of the transfer material.

The transfer material on which toner images are transferred by therespective printer engines are discharged outside the apparatus bydischarge roller pairs 59-a and 59-b. Note that the feeding belt 10 maybe an intermediate feeding belt which has an arrangement for temporarilytransferring C (CYAN), Y (YELLOW), M (MAGENTA), and K (BLACK) colortoners, and then secondarily transferring them onto a transfer material.

FIG. 3 is a conceptual view for explaining a misalignment of a main scanline scanned on each photosensitive drum 14 as an image carrier.

Reference numeral 201 denotes an image of an ideal main scan line whichis scanned in a direction perpendicular to the rotational direction ofeach photosensitive drum 14 (the longitudinal direction of the drum=themain scan direction). Reference numeral 202 denotes an image of anactual main scan line which has suffered a right upward inclination andcurvature resulting from errors of the positional precision and diameterof each photosensitive drum 14, and the positional precision error ofthe exposure unit 51 of each color. If the printer engine of any colorsuffers such inclination and curvature of the main scan line, colordiscrepancy occurs upon simultaneously transferring toner images of aplurality of colors onto a transfer material.

In this embodiment, the discrepancy amounts of the actual main scan line202 in the sub-scan direction with respect to the ideal main scan line201 are measured at a plurality of points (points B, C, and D) to havepoint A as a reference point, which serves as the scan start position ofthe print region in the main scan direction (X-direction). The main scanline is divided into a plurality of regions (region 1 between Pa and Pb,region 2 between Pb and Pc, and region 3 between Pc and Pd in theexample shown in FIG. 3) in correspondence with the points where thediscrepancy amounts are measured, and the inclinations of main scanlines of the respective regions are approximated by straight lines (Lab,Lbc, and Lcd) that connect neighboring points. Therefore, when adifference (m1 for region 1, m2−m1 for region 2, or m3−m2 for region 3)between the discrepancy amounts of neighboring points assumes a positivevalue, it Indicates that the main scan line of the region of interesthas a right upward inclination; when the difference assumes a negativevalue, it indicates that the main scan line of that region has a rightdownward inclination. Note that the unit of m1, m2, m3, L1, L2, and L3in FIG. 3 is dots.

FIG. 4 is a control block diagram associated with color discrepancycorrection processing according to this embodiment.

A printer engine 50 executes print processing based on bitmap data to beprinted generated by a controller 402.

The printer engine 50 comprises color discrepancy amount storage units40C, 40M, 40Y, and 40K which respectively store the aforementioneddiscrepancy amounts of main scan lines for respective regions. In thisembodiment, each of the color discrepancy amount storage units 40C, 40M,40Y, and 40K stores, as information of the color discrepancy amount, thediscrepancy amounts between the actual main scan line 202 and ideal mainscan line 201 in the sub-scan direction, which are measured at theplurality of points described in FIG. 3. FIG. 5 shows an example ofinformation to be stored in each of the color discrepancy amount storageunits 40C, 40M, 40Y, and 40K.

In this embodiment, each of the color discrepancy amount storage units40C, 40M, 40Y, and 40K stores the discrepancy amounts between the idealmain scan line and actual main scan line. However, the present inventionis not limited to this as long as the degree of the inclination orcurvature of the actual main scan line is identifiable information. Asinformation to be stored in each of the color discrepancy amount storageunits 40C, 40M, 40Y, and 40K, the above discrepancy amounts may bemeasured in the manufacturing process of the apparatus, and may bepre-stored as information unique to the apparatus. Alternatively, adetection mechanism that detects the discrepancy amounts may be preparedto form a predetermined pattern used to measure discrepancy for an imagecarrier of each color, and the discrepancy amounts detected by thedetection mechanism may be stored.

The controller 402 in FIG. 4 executes print processing by correctingimage data to cancel the discrepancy amounts of the main scan linesstored in the color discrepancy amount storage units 40C, 40M, 40Y, and40K.

More specifically, an image generating unit 404 generates raster imagedata, which allows print processing, based on print data received from acomputer apparatus or the like (not shown), and outputs that data as RGBdata for respective dots. A color conversion unit 405 converts the RGBdata into data on a CMYK space which can be processed by the printerengine 50, and stores the converted data in a bitmap memory 406 (to bedescribed below) for respective colors. The bitmap memory 406temporarily stores the raster image data to be printed, and is a pagememory that can store image data for one page or a band memory that canstore data for a plurality of lines.

Reference numerals 407C, 407M, 407Y, and 407K denote color discrepancycorrection position arithmetic units. The color discrepancy correctionposition arithmetic units calculate positions where coordinateconversion is to be made later as first color discrepancy correction,based on the information of the color discrepancy amounts stored in thecolor discrepancy amount storage units 40C, 40M, 407, and 40K. Also, thecolor discrepancy correction position arithmetic units calculatepositions where the tone correction level is to be switched as secondcolor discrepancy correction. The calculation results are output tocorresponding color discrepancy correcting units 408C, 408M, 408Y, and408K.

An example of the arithmetic contents of respective regions based onFIG. 3 by the color discrepancy correction position arithmetic units407C, 407M, 407Y, and 407K will be described below.

In this embodiment, the coordinate conversion to be described later isdone at the following positions.

Region 1: The coordinate conversion is done every (L1/m1) dots in themain scan direction.

Region 2: The coordinate conversion is done every (L2−L1)/(m2−m1) dotsin the main scan direction.

Region 3: The coordinate conversion is done every (L3−L2)/(m3−m2) dotsin the main scan direction.

Thus, the color discrepancy correction position arithmetic units 407C,407M, 407Y, and 407K respectively calculate (L1/m1), (L2−L1)/(m2−m1),and (L3−L2)/(m3−m2). These values respectively correspond to thereciprocal numbers of the inclinations of the actual main scan lines inrespective regions.

Also, in this embodiment, the density of the tone correction to bedescribed later is switched to, e.g., three levels as follows.

Region 1: The density of the tone correction is switched every((L1/m1)/3) dots in the main scan direction.

Region 2: The density of the tone correction is switched every(((L2−L1)/(m2−m1))/3) dots in the main scan direction.

Region 3: The density of the tone correction is switched every(((L3−L2)/(m3−m2))/3) dots in the main scan direction.

Hence, the color discrepancy correction position arithmetic units 407C,407M, 407Y, and 407K respectively calculate (L1/m1)/3,((L2−L1)/(m2−m1))/3, and ((L3−L2)/(m3−m2))/3.

In this way, the color discrepancy correction position arithmetic units407C, 407M, 407Y, and 407K calculate the positions where the coordinateconversion is to be done and those where the density of the tonecorrection is to be switched in correspondence with the inclinations ofthe actual main scan lines.

L1, L2, and L3 are distances (unit: dots) from the print start positionto the left ends of regions 1, 2, and 3 in the main scan direction m1,m2, and m3 are discrepancy amounts (unit: dots) between the ideal mainscan line 301 and actual scan line 302 at the left ends of regions 1, 2,and 3.

The color discrepancy correcting units 408C, 408M, 408Y, and 408K shownin FIG. 4 correct color discrepancy due to the inclinations anddistortions of the main scan lines. More specifically, the colordiscrepancy correcting units 408C, 408M, 408Y, and 408K adjust theoutput timings of the bitmap data stored in the bitmap memory 406 andadjust the exposure amounts for respective pixels on the basis of thecolor discrepancy correction positions calculated by the colordiscrepancy correction position arithmetic units 407C, 407M, 407Y, and407K. In this manner, any color discrepancy (registration errors) upontransferring toner images of respective colors onto a transfer materialcan be prevented.

Each of the color discrepancy correcting units 408C, 408M, 408Y, and408K has an arrangement shown in FIG. 4 in principle. For example, thecolor discrepancy correcting unit 408C comprises a coordinate counter801C, coordinate converter 802C, line buffer 803C, tone corrector 804C.The coordinate counter 801C outputs coordinate data in the main scandirection and sub-scan direction where color discrepancy correctionprocessing is to be executed to the coordinate converter 802C, and alsothe coordinate data in the main scan direction to the tone corrector804C. The coordinate converter 802C executes reconstruction processingin the sub-scan direction for respective pixels based on the coordinatedata in the main scan direction and sub-scan direction from thecoordinate counter 801C and the correction position obtained from thecolor discrepancy correction position arithmetic unit 407C. The tonecorrector 804C performs correction in less than the pixel unit usingpredetermined exposure ratios of several levels in the sub-scandirection based on the coordinate data in the main scan direction fromthe coordinate counter 801C and the correction position obtained fromthe color discrepancy correction position arithmetic unit 407C. Also,the tone corrector 804C uses the line buffer 803C to refer toneighboring dots in the sub-scan direction. The other color discrepancycorrecting units 408M, 408Y, and 408K have the same arrangement.

FIGS. 6A to 6C are views for explaining discrepancy amount correction inthe pixel unit by the coordinate converter 802.

The coordinate converter 802 offsets coordinates in the sub-scandirection (Y-direction) of the bitmap data to be printed stored in thebitmap memory 406 for respective color discrepancy correction positionscalculated based on the color discrepancy information of the main scanlines approximated by straight lines, as shown in FIG. 6A (i.e., in adot count unit in the main scan direction according to the colordiscrepancy amounts).

For example, as shown in FIG. 6B, if the coordinate in the sub-scandirection from the coordinate counter 801 is n, letting X be thecoordinate in the main scan direction, the color discrepancy correctionamount in region (1) is zero. At this time, upon reconstructing data ofthe n-th line, the data of the n-th line is read out from the bitmapmemory. In region (2), the color discrepancy correction amount is 1, andupon reconstructing data of the n-th line, coordinate conversionprocessing for reading out an image bitmap at a position offset by onesub-scan line count, i.e., data of the (n+1)-th line from the bitmapmemory is executed. Likewise, coordinate conversion processing forreading out data of the (n+2)-th line for region (3) and that forreading out data of the (n+3)-th line for region (4) are executed. Withthe aforementioned method, the reconstruction processing in the sub-scandirection in the pixel unit is executed.

FIG. 6C shows an exposed image formed by exposing image data which hasundergone the color discrepancy correction in the pixel unit by thecoordinate converter 802 on the photosensitive drum.

FIGS. 7A to 7G are views for explaining color discrepancy correction inless than the pixel unit by the tone corrector 804. The discrepancyamount less than the pixel unit is corrected adjusting the exposureratios of neighboring dots in the sub-scan direction.

FIG. 7A shows an image of a main scan line having a right upwardinclination. FIG. 7B shows a bitmap image of a horizontal straight linebefore coordinate conversion. FIG. 7C shows a bitmap image before tonecorrection. FIG. 7D shows a correction image of FIG. 7B to cancel colordiscrepancy due to the inclination of the main scan line in FIG. 7A. Inorder to realize the correction image in FIG. 7D, neighboring dots inthe sub-scan direction undergo exposure amount adjustment. FIG. 7E showsthe relationship between k which represents a correction amount in thesub-scan direction in the pixel unit, and correction coefficients α andβ used to tone correction. α and β are correction coefficients used toperform correction in less than the pixel unit in the sub-scandirection, and indicate distribution ratios of the density (exposureamount) to neighboring dots in the sub-scan direction. For example, ifthe distribution ratios of three levels are prepared, α and β are:

First level:

α=0

β=1

Second level:

α=0.333

β=0.666

Third level:

α=0.666

β=0.333

(β+α=1). α is the distribution ratio of the previous dot, and β is thatof the next dot. The level of the distribution ratio is switched basedon the tone correction position information in the main scan direction,which is calculated by the color discrepancy correction positionarithmetic unit 407.

FIG. 7F shows a bitmap image which has undergone tone correction toadjust the exposure ratios of neighboring dots in the sub-scan directionin accordance with the correction coefficients shown in FIG. 7E. FIG. 7Gshows an exposed image of the bitmap image that has undergone the tonecorrection on the photosensitive drum. In FIG. 7G, the inclination ofthe main scan line is canceled, and a nearly horizontal straight line isformed.

FIG. 8 is a block diagram showing the detailed arrangement of the colordiscrepancy correcting units 408C, 408M, 408Y, and 408K. The method ofgenerating a correction bitmap by the tone correction processing will bedescribed below with reference to FIG. 8.

The coordinate converter 802 transfers image bitmap data which isreconstructed to correct the color discrepancy amounts in the pixel unitby the bitmap memory 406 to the line buffer 803.

The tone corrector 804 uses the line buffer 803 for one line to refer tothe previous and next pixel values in the sub-scan direction so as togenerate correction data. The line buffer 803 includes a FIFO (first infirst out) buffer 806 which stores data for one line of the previousline, and a register 805 which holds pixel data of the coordinate whichis to undergo tone correction processing. The pixel data stored in theregister 805 is output to the tone corrector 804, and is stored in theFIFO buffer 806 since it is used to generate correction data for thenext line. In order to generate correction data, the tone corrector 804executes arithmetic processing given by:

P′ _(n)(x)=P _(n)(x)*β(x)+P _(n-1)(x)*α(x)  (1)

where x (dots) is the coordinate in the main scan direction, P_(n)(x) ispixel data input from the register 805, and P_(n-1)(x) is the pixel datainput from the FIFO buffer 806.

With the above arithmetic processing, an image bitmap in which the colordiscrepancy amount in the sub-scan direction less than the pixel unit iscorrected is output.

The image data that has undergone the color discrepancy correction bythe above processing undergoes halftone processing using a predeterminedhalftone pattern by each of next halftone processing units 409C, 409M,409Y, and 409K. The image data undergoes pulse width modulationprocessing by each of PWM units 410C, 410M, 410Y, and 410K, and isoutput to the printer engine 50, thus performing exposure processing onthe photosensitive drum 14 as an image carrier.

As described above, the correction position required to correct thediscrepancy amount in the sub-scan direction at each main scan positionis calculated from an image bitmap, and a corrected image bitmap isreconstructed according to the correction position, thus generating animage free from any color discrepancy due to the inclination anddistortion of the main scan line.

As for the distribution ratios α and β of the exposure amount ofneighboring dots in the sub-scan direction, for example, if two levelsof distribution ratios are prepared, α and β are:

First level:

α=0

β=1

Second level:

α=0.5

β=0.5

Multiplication by 0.5 is equivalent to right shift. Hence, thearithmetic processing given by equation (1) above by the tone corrector804 can be implemented by only bit shift. By replacing multipliers byshifters, an image free from any color discrepancy due to theinclination and distortion of the main scan line can be generated by asimpler processing system.

Alternatively, as for the distribution ratios α and β of the exposureamount of neighboring dots in the sub-scan direction, for example, iffour levels of distribution ratios are prepared, α and β are:

First level:

α=0

β=1

Second level:

α=0.25

β=0.75

Third level:

α=0.5

β=0.5

Fourth level:

α=0.75

β==0.25

Multiplication by 0.5 is equivalent to right shift. Also, multiplicationby 0.25 is equivalent to right shift by 2 bits. Furthermore,multiplication by 0.75 is the sum of 0.5 and 0.25. Hence, the arithmeticprocessing given by equation (1) above by the tone corrector 804 can beimplemented by only bit shift and addition. By replacing multipliers byshifters and an adder, an image free from any color discrepancy due tothe inclination and distortion of the main scan line can be generated bya simpler processing system.

According to the aforementioned first embodiment, each color discrepancycorrection position arithmetic unit calculates the color discrepancycorrection position based on the color discrepancy amount due to theinclination and distortion (e.g., curvature or the like) of the scanline that scans the photosensitive drum as an image carrier, which isheld in the color discrepancy amount storage unit. Each colordiscrepancy correcting unit reconstructs an image bitmap by performingcolor discrepancy correction in the pixel unit and that of severallevels (e.g., 3 levels) less than the pixel unit using the correctioncoefficients α and β of fixed values. In this manner, color discrepancydue to the inclination, curvature, and the like of the main scan linethat exposes the photosensitive drum can be prevented by processingsimpler than the arrangement that makes optical correction, thusobtaining a high-quality color image.

Furthermore, by applying color discrepancy correction in less than thepixel unit in two or four levels, the processing system can be moresimplified.

Second Embodiment

In the second embodiment, when the density of the tone correction isswitched at, e.g., three pixels in four levels, it is done at (a) aposition where coordinate conversion is performed, (b) a position onepixel before the position where coordinate conversion is performed, and(c) a position two pixels before the position where coordinateconversion is performed.

Hence, the color discrepancy correction position arithmetic units 407C,407M, 407Y, and 407K respectively calculate the position one pixelbefore the position where coordinate conversion is performed, and theposition two pixels before the position where coordinate conversion isperformed.

FIGS. 9A to 9G are views for explaining color discrepancy correction inless than the pixel unit by the tone corrector 804. The discrepancyamount correction in less than the pixel unit is implemented byadjusting the exposure ratios of neighboring dots in the sub-scandirection.

FIG. 9A shows an image of a main scan line having a right upwardinclination. FIG. 9B shows a bitmap image of a horizontal straight linebefore coordinate conversion. FIG. 9C shows a bitmap image before tonecorrection. FIG. 9D shows a correction image of FIG. 9B to cancel colordiscrepancy due to the inclination of the main scan line in FIG. 9A. Inorder to realize the correction image in FIG. 9D, neighboring clots inthe sub-scan direction undergo exposure amount adjustment. FIG. 9E showsthe relationship between k which represents a correction amount in thesub-scan direction in the pixel unit, and correction coefficients α andβ used to tone correction. α and β are correction coefficients used toperform correction in less than the pixel unit in the sub-scandirection, and indicate distribution ratios of the density (exposureamount) to neighboring dots in the sub-scan direction. For example, inorder to perform tone correction at four pixels, distribution ratios offive levels must be prepared, and more specifically, α and β are:

First level:

α=0

β=1

Second level:

α=0.2

β=0.8

Third level:

α=0.4

β=0.6

Fourth level:

α=0.6

β=0.4

Fifth level

α=0.8

β=0.2

(β+α=1). α is the distribution ratio of the previous dot, and β is thatof the next dot. The level of the distribution ratio is switched at fourpixels near the pixel where the coordinate conversion is done, based onthe tone correction position information in the main scan direction,which is calculated by the color discrepancy correction positionarithmetic unit 407.

FIG. 9F shows a bitmap image which has undergone tone correction toadjust the exposure ratios of neighboring dots in the sub-scan directionin accordance with the correction coefficients shown in FIG. 9E. FIG. 9Gshows an exposed image of the bitmap image that has undergone the tonecorrection on the photosensitive drum. In FIG. 9G, the inclination ofthe main scan line is canceled, and a nearly horizontal straight line isformed.

As described above, the correction position required to correct thediscrepancy amount in the sub-scan direction at each main scan positionis calculated from an image bitmap, and a corrected image bitmap isreconstructed according to the correction position, thus generating animage free from any color discrepancy due to the inclination anddistortion of the main scan line.

For example, if distribution ratios of four levels are prepared toexecute tone correction at three pixels, α and β are:

First level:

α=0

β=1

Second level:

α=0.25

β=0.75

Third level:

α=0.5

β=0.5

Fourth level:

α=0.75

β=0.25

Multiplication by 0.5 is equivalent to right shift. Also, multiplicationby 0.25 is equivalent to right shift by 2 bits. Furthermore,multiplication by 0.75 is the sum of 0.5 and 0.25. Hence, the arithmeticprocessing given by equation (1) above by the tone corrector 804 can beimplemented by only bit shift and addition. By replacing multipliers byshifters and an adder, an image free from any color discrepancy due tothe inclination and distortion of the main scan line can be generated bya simpler processing system.

Ad described above, according to the second embodiment, the same effectsas in the first embodiment can be obtained. That is, color discrepancydue to the inclination, curvature, and the like of the main scan linethat exposes the photosensitive drum can be prevented by processingsimpler than the arrangement that makes optical correction, thusobtaining a high-quality color image.

Third Embodiment

According to parent reference 2 (Japanese Patent Application Laid-OpenNo. 8-85237) described above, the output coordinate position of imagedata for each color is corrected for an image that has undergonehalftone processing. For this reason, if dithering is applied,reproducibility of halftone dots of a halftone image deteriorates. As aresult, color inconsistency may occur and moiré may become obvious.Furthermore, when such non-uniform density values are periodicallyrepeated, moiré becomes obvious, and a high-quality color image cannotbe obtained. The third embodiment solves such drawbacks.

A color image forming apparatus according to the embodiment of thepresent invention is also a four-drum color laser beam printer, and FIG.2 will be quoted.

FIG. 10 is a conceptual view for explaining a misalignment of a mainscan line scanned on each photosensitive drum 14 as an image carrier(for example, the photosensitive drum 14-C for cyan). Since the sameapplies to photosensitive drums corresponding to other colors, adescription thereof will, be omitted.

Reference numeral 201 denotes an image of an ideal main scan line whichis scanned in a direction perpendicular to the rotational direction ofeach photosensitive drum 14-C (the longitudinal direction of the drum).Reference numeral 202 denotes an image of a main scan line which hassuffered a right upward inclination and curvature by an actual laserscan, which occur due to errors of the positional precision and diameterof each photosensitive drum 14-C, and the positional precision error ofan optical system of the cyan exposure unit 51-C. If the image stationof any color suffers such inclination and curvature of the main scanline, color discrepancy occurs upon simultaneously transferring tonerimages of a plurality of colors onto a transfer material.

In this embodiment, the discrepancy amount in the sub-scan directionbetween the ideal main scan line 201 and actual main scan line 202 ismeasured at a plurality of points (points B, C, and D) to have point Aas a reference point, which serves as the scan start position of theprint region in the main scan direction (x-direction: the longitudinaldirection of the drum). The measured discrepancy amount is divided intoa plurality of regions (region 1 between Pa and Pb, region 2 between Pband Pc, and region 3 between Pc and Pd) in correspondence with themeasurement points, and the inclinations of main scan lines of therespective regions are approximated by straight lines (Lab, Lbc, andLcd) that connect neighboring points. Therefore, when a difference (m1for region 1, (m2−m1) for region 2, or (m3−m2) for region 3) between thediscrepancy amounts of neighboring points Pa, Pb, Pc, and Pd assumes apositive value, the main scan line of the region of interest has a rightupward inclination; when the difference assumes a negative value, themain scan line of that region has a right downward inclination. FIG. 10is a view similar to FIG. 3. Note that the unit of m1, m2, m3, L1, L2,and L3 in FIG. 10 is mm.

FIG. 11 is a block diagram for explaining color discrepancy correctionprocessing for correcting color discrepancy that occurs due to theinclination and curvature of the main scan line in this embodiment.

Reference numeral 301 denotes a printer engine which has image formingunits shown in FIG. 2, and executes print processing based on bitmapimage data generated by a controller 302. Reference numerals 303C, 303Y,303M, and 303K denote color discrepancy amount storage units whichrespectively store color discrepancy amounts for respective colors,i.e., cyan, yellow, magenta, and black. These units store discrepancyamounts of the main scan lines for the respective regions describedabove in correspondence with colors. In practice, a misalignment amountof an Image to be formed is stored. However, this amount causes colordiscrepancy, and it will be referred to as a “color discrepancy amount”hereinafter. In this embodiment, discrepancy amounts in the sub-scandirection with respect to the ideal main scan line 201 based on thepositions of the actual main scan line 202 measured at the plurality ofpoints, as described above using FIG. 10, are stored as informationindicating the inclination and curvature of the main scan line 202 inthe color discrepancy amount storage unit 303.

FIG. 12 shows an example of data to be stored in this color discrepancyamount storage unit 303 (303C, 303Y, 303M, and 303K).

In FIG. 12, the lengths (L1, L2, and L3) in the main scan direction fromthe reference point to the actual measurement points on the main scanline 202, and discrepancy amounts (m1, m2, and m3) between the points(Pb, Pc, and Pd) on the main scan line 202 and the ideal main scan line201 are stored in association with each other. Note that the unit of L1,L2, L3, m1, m2, and m3 is mm. L1, L2, and L3 respectively representlengths from the reference point (point A) to the terminal ends ofregions 1, 2, and 3. Also, m1, m2, and m3 are discrepancy amountsbetween the ideal main scan line 201 and actual main scan line 202 atthe terminal ends of regions 1, 2, and 3 (see FIG. 10).

In this embodiment, each of the color discrepancy amount storage units303C, 303M, 303Y, and 303K stores the discrepancy amounts between theideal main scan line 201 and actual main scan line 202 on thephotosensitive drum. However, the present invention is not limited tothis as long as the characteristics of the inclination and curvature ofthe actual main scan line 202 are identifiable information. Asinformation to be stored in each of the color discrepancy amount storageunit 303, the above discrepancy amounts may be measured in themanufacturing process of the apparatus, and may be pre-stored asinformation unique to the apparatus. Alternatively, a detectionmechanism that detects the discrepancy amounts may be prepared to form apredetermined pattern used to measure discrepancy for an image carrier(photosensitive drum) of each color, and the discrepancy amountsdetected by the detection mechanism may be stored.

An operation for executing print processing by correcting image data tocancel the discrepancy amounts of the main scan lines stored in thecolor discrepancy amount storage unit 303 in the controller 302 will bedescribed below.

More specifically, an image generating unit 304 generates raster imagedata, which allows print processing, based on print data received froman external apparatus (not shown) such as a computer apparatus or thelike, and outputs that data as RGB data for respective dots. A colorconversion unit 305 converts the RGB data into data on a CMYK spacewhich can be processed by the printer engine 301, and stores theconverted data in a bitmap memory 306 (to be described below) forrespective colors. The bitmap memory 306 temporarily stores the rasterimage data to be printed, and comprises either a page memory that canstore image data for one page or a band memory that can store data for aplurality of lines.

Reference numerals 307C, 307Y, 307M, and 307K denote color discrepancyamount arithmetic units which calculate correction amounts of colordiscrepancy corresponding to respective color data. These arithmeticunits calculate color discrepancy correction amounts in the sub-scandirection corresponding to coordinate information in the main scandirection designated by color discrepancy correcting units 308 (to bedescribed later) for respective dots on the basis of informationindicating the discrepancy amounts of the main scan lines stored in thecolor discrepancy amount storage units 303 corresponding to respectivecolors. The calculation results are output to the corresponding colordiscrepancy correcting units 308.

Let x (dots) be a coordinate of a given dot in the main scan direction,y (lines) be a coordinate of that dot in the sub-scan direction, and Δyi(dots) (i indicates a region) be the color discrepancy correction amountin the sub-scan direction. In this case, arithmetic expressions of thecolor discrepancy correction amounts Δyi in the sub-scan direction inrespective regions based on FIG. 10 are as follows (note that theresolution in this case is 600 dpi).

Region 1: Δy1=x×(m1/L1)  (2)

Region 2: Δy2=m1×23.622+(x−L1×23.622)×((m2−m1)/(L2−L1))  (3)

Region 3: Δy3=m2×23.622+(x−L2×23.622)×((m3−m2)/(L3−L2))  (4)

The color discrepancy correcting units 308C, 308Y, 308M, and 308Krespectively correct color discrepancy due to the inclinations andcurvatures of the main scan lines. More specifically, these correctingunits adjust the output timings of bitmap data stored in the bitmapmemory 306 and adjust the exposure amounts for respective dots based onthe color discrepancy correction amounts calculated for respective dotsby the color discrepancy correction amount arithmetic units 307C, 307Y,307M, and 307K. In this manner, any color discrepancy (registrationerrors) upon transferring toner images of respective colors onto atransfer sheet can be prevented.

Each color discrepancy correcting unit 308 according to this embodimentwill be described below with reference to the block diagram shown inFIG. 13.

FIG. 13 is a block diagram showing the arrangement of the colordiscrepancy correcting unit 308C according to this embodiment. Sinceother color discrepancy correcting units 308Y, 308M, and 308K have thesame arrangement, a description of the correcting units 308Y, 308M, and308K corresponding to other colors will be omitted.

The color discrepancy correcting unit 308C comprises a coordinatecounter 701, coordinate converter 702, line buffer 703, and tonecorrector 704. The coordinate counter 701 outputs coordinate data (x, y)in the main scan direction and sub-scan direction of a dot, that is toundergo color discrepancy correction processing, to the coordinateconverter 702. At the same time, the coordinate counter 701 outputscoordinate data x in the main scan direction to the color discrepancycorrection amount arithmetic unit 307C and tone corrector 704. Thecoordinate converter 702 executes correction processing of the integerpart of a correction amount Δy based on the coordinate data (x, y) inthe main scan direction and sub-scan direction from the coordinatecounter 701 and the correction amount Δy obtained from the colordiscrepancy correction amount arithmetic unit 307C. That is, thecoordinate converter 702 executes reconstruction processing in thesub-scan direction for respective dots.

The tone corrector 704 performs correction processing of the decimalpart of the correction amount Δy based on the coordinate data x in themain scan direction from the coordinate counter 701 and the correctionamount Δy obtained from the color discrepancy correction amountarithmetic unit 307C. That is, as for a correction amount less than adot unit, the tone corrector 704 performs correction by adjusting theON/OFF ratios of corresponding dots on neighboring lines in the sub-scandirection with respect to data on the current line. Also, the tonecorrector 704 uses the line buffer 703 to refer to neighboring dots inthe sub-scan direction.

FIG. 14 is a conceptual view for explaining the operation contents whenthe coordinate converter 702 according to this embodiment corrects thediscrepancy amount of the integer part of the color discrepancycorrection amount Δy, i.e., color discrepancy for each line.

The coordinate converter 702 offsets a coordinate of image data (cyan inthis case) in the sub-scan direction (Y-direction), which is stored inthe bitmap memory 306, in accordance with the value of the integer partof the color discrepancy correction amount Δy calculated based on thecolor discrepancy information of the main scan lines approximated bystraight lines, as indicated by reference numeral 600. For example, letn (lines) be a coordinate of the position of dot data 610, as indicatedby reference numeral 601. This value is obtained from the coordinatecounter 701. Also, let x be a coordinate of that dot data in the mainscan direction. Then, the color discrepancy correction amount Δy1 inregion (1) satisfies 0≦Δy1<1. Hence, upon reconstructing the data 610whose coordinate in the sub-scan direction in region (1) is n, data ofthe n-th line is read out from the bitmap memory 306.

In region (2), the color discrepancy correction amount Δy2 satisfies1≦Δy2<2. Therefore, upon reconstructing data, coordinate conversionprocessing for reading out an image bitmap at a position offset by 1 asthe number of sub-scan lines, i.e., data of the (n+1)-th line from thebitmap memory 306 is executed. Likewise, coordinate conversionprocessing for reading out data of the (n+2)-th line for region (3) andthat for reading out data of the (n+3)-th line for region (4) areexecuted.

With the aforementioned method, the reconstruction processing in thesub-scan direction in a dot unit is executed.

Reference numeral 602 denotes an exposed image obtained by exposingimage data which has undergone color discrepancy correction in a dotunit by the coordinate converter 702 on the photosensitive drum 14C.

FIG. 15 is a conceptual view for explaining the operation contents ofcolor discrepancy correction in less than a dot unit executed by thetone corrector 704 according to this embodiment, i.e., those forcorrecting the discrepancy amount of the decimal part of the colordiscrepancy correction amount Δy.

Referring to FIG. 15, reference numeral 720 denotes a dot distribution(correction amount) on the Current line (n-th line); and 721, a dotdistribution (correction amount) on the next line ((n+1)-th line). Inthis way, in this embodiment, the discrepancy amount of a fraction belowthe decimal point is corrected by adjusting the ON/OFF ratios of dots onlines located before or after the current line in the sub-scandirection. In FIG. 15, the inclination discrepancy amount is 1 dot/48dots. In this embodiment, color discrepancy correction in less than adot unit is done by dividing this 48-dot section into six regions(regions (1) to (6), and each region is delimited by 8 dots. At thistime, all 8 dots are ON on only the n-th line in region (1). In region(2), 6 dots are ON on the n-th line and 2 dots are ON on the (n+1)-thline. In regions (3) and (4), 4 dots are ON on each of the n-th and(n+1)-th lines. Furthermore, in region (5), 2 dots are ON on the n-thline and the remaining 6 dots are ON on the (n+1)-th line. In region(6), all 8 dots are ON on the (n+1)-th line. In this way, colordiscrepancy correction in less than a dot unit is done.

In this embodiment, the number of divided correction regions is six.However, the present invention is not limited to such specific value.For example, even when the inclination or discrepancy amount is anindivisible value, tone correction can be made by assigning remainingdots to an arbitrary region.

This operation will be described below with reference to the blockdiagram of the color discrepancy correcting unit shown in FIG. 13.

The coordinate converter 702 transfers bitmap data which isreconstructed to correct color discrepancy amounts for respective dotsfrom the bitmap memory 306 to the line buffer 703. The tone corrector704 uses the line buffer 703 for one line so as to refer to dot valuesbefore and after the current line (n-th line) in the sub-scan direction.The line buffer 703 includes a FIFO (first in first out) buffer 706which stores data for one line of the previous line, and a register 705which holds dot data of the coordinate which is to undergo tonecorrection processing. The dot data stored in the register 705 is outputto the tone corrector 704, and is stored in the FIFO buffer 706 since itis used to generate correction data for the next line. The tonecorrector 704 determines the current region based on the coordinate x(dots) in the main scan direction, and determines tone to be output. Forexample, in case of a coordinate in region (4) in FIG. 15, the tonecorrector 704 expresses tone by alternately outputting dot data Pn(x) ofthe n-th line and dot data Pn−1(x) of the previous line.

In the above description, correction processing implemented by hardwarehas been explained. When the controller 302 comprises a CPU, processingcan also be implemented by software.

FIG. 16 is a block diagram showing an example of the controller 302shown in FIG. 11, which is configured by a CPU and memories. The samereference numerals in FIG. 16 denote components common to those in FIG.11 above, and a description thereof will be omitted.

The printer engine 301 has the same arrangement as in FIG. 11, and theexposure units 51, photosensitive drums 14, and the like are notillustrated. The color discrepancy amount storage units 303C to 303Krespectively store the color discrepancy amounts on the photosensitivedrums 14C to 14K corresponding to respective colors, as described above.The controller 302 comprises a CPU 1000, a ROM 1001 which storesprograms to be executed by the CPU 1000 and various data, and a RAM 1002which is used as a work area in control processing by the CPU 1000, andtemporarily saves various data. This RAM 1002 has the bitmap memory 306which stores cyan, yellow, magenta, and black bitmap image data. On theRAM 1002, areas 1010 for storing color discrepancy data which areacquired from the color discrepancy amount storage units 303C to 303Kand correspond to respective colors are also assured.

FIGS. 17 and 18 are flowcharts for explaining image forming processingto be executed by the CPU 1000 of the controller 302 according to thisembodiment. A program that implements this processing is stored in theROM 1001, and is executed under the control of the CPU 1000.

In step S1, the color discrepancy amounts for respective colors storedin the color discrepancy amount storage units 303C to 303K of theprinter engine 301 are read out, and are stored in the areas 1010 on theRAM 1002. In step S2, print data is input and undergoes processing suchas color conversion and the like. After that, the print data isconverted into cyan, yellow, magenta, and black bitmap image data eachfor one page, and the converted data are stared in the bitmap memory306. In step S3, a variable n used to count the number of lines is resetto “1”, and a variable x used to count the dot position (x coordinate)is reset to “0”. Note that both these variables are stored in the RAM1002.

In step S4, the x-th dot data of the n-th line is read out from the cyanbitmap data. In step S5, a region which includes that dot (for example,one of regions 1 to 3 in FIG. 10) is determined. In step S6, acorrection amount Δy in the sub-scan direction, which forms that dot iscalculated based on the region determined in step S5 and the dotposition (x). This value can be calculated using one of equations (2) to(4) above. It is checked in step S7 if the Integer part of thecorrection amount Δy calculated in step S6 is “0”. If the integer partis “0”, since correction in a line unit is not required, the flow jumpsto step S11; otherwise, the flow advances to step S8 to check if theinteger part is positive or negative. If the integer part is positive,the flow advances to step S9 to acquire the x-th dot data of the(n+s)-th line and specify it as the dot data of the current line (seeFIG. 14). On the other hand, if it is determined in step S8 that theinteger part is negative, the flow advances to step S10 to the x-th dotdata of the (n−s)-th line and specify it as the dot data of the currentline (see FIG. 14), Note that s indicates the absolute value of thatinteger part. After step S9 or S10, the flow advances to step S11.

In step S11, processing for the numerical value of the decimal part ofthe correction amount Δy is executed in turn. According to the numericalvalue of the decimal part, the distribution of the x-th dot data on thecurrent line (n-th line) and the (n+1)-th or (n−1)-th line isdetermined. As has been described above with reference to FIG. 15, dotdata are exchanged or replaced between those on neighboring lines inaccordance with the numerical value of the decimal part. In this manner,after the x-th dot data on the current line (n-th line) is updated, thebitmap data is updated in step S12. In step S13, the variable x isincremented by +1. It is checked in step S14 if the value of thevariable x becomes larger than the total number of dots of one line. Ifthe value of the variable x is smaller than the total number of dots,the flow returns to step S4 to repeat the aforementioned processing.

If it is determined in step S14 that the value of the variable x becomeslarger than the total number of dots of one line, the flow advances tostep S15 to increment the variable n used to count the number of linesby +1. It is then checked in step S16 if the value of this variable nexceeds the number of lines of one page. If the value of this variable ndoes not exceed the number of lines of one page, the flow advances tostep S17 to reset the variable x to “0”. The flow then returns to stepS4 to repeat the aforementioned processing. On the other hand, If it isdetermined in step S16 that the value of the variable n exceeds thenumber of lines of one page, the flow advances to step S18. It ischecked in step S18 if processing for the cyan, yellow, magenta, andblack bitmap data is complete. If the processing is not complete yet,the flow returns to step S3 to repeat the aforementioned processing;otherwise, the flow advances to step S19 to start image formingprocessing.

In step S19, a transfer sheet is picked up from the cassette 53 andbegins to be fed. While the transfer sheet is fed placed on the feedingbelt 10, toner images are formed in turn in the order of cyan, yellow,magenta, and black (step S20), and they are transferred in turn on thefed transfer sheet (step S21). Upon completion of transfer, the tonerimages are fixed on the transfer sheet in step S22. Upon completion offixing, the fixed transfer sheet is discharged in step S23.

As described above, with the color image forming apparatus according tothis embodiment, both color discrepancy in a dot unit and that in anamount less than a dot unit can be corrected based on the colordiscrepancy amounts on respective photosensitive drums. In this way,color discrepancy of respective color images due to the inclinations,curvatures, and the like of scan lines that scan and expose therespective photosensitive drums can be prevented, thus obtaining ahigh-quality color image.

Fourth Embodiment

According to parent reference 2 (Japanese Patent Application Laid-OpenNo. 8-85237) described above, the output coordinate position of imagedata for each color is corrected for an image that has undergonehalftone processing. For this reason, if dithering is applied,reproducibility of halftone dots of a halftone image deteriorates. As aresult, color inconsistency may occur and moiré may become obvious, ashas been described previously. That is, when image data having constantdensity values is input and that image data undergoes the aforementionedcolor discrepancy correction and is printed, the following problems mayoccur. In general, the density value of image data and a toner densityfor that density value do not have a linear relationship. For thisreason, although the input image data has constant density values, if itis corrected in an amount less than a minimum dot unit, the density ofthe corrected image is no longer constant. When such non-uniform densitypart is periodically repeated, moiré becomes obvious, and a high-qualitycolor image cannot be obtained. The fourth embodiment solves suchdrawbacks.

The arrangement of the color image forming apparatus shown in FIG. 2will be quoted in this embodiment.

FIG. 19 is a block diagram for explaining color discrepancy correctionprocessing for correcting color discrepancy that occurs due to theinclination and curvature of the main scan line in this embodiment.

Reference numeral 301 denotes a printer engine which has image formingunits shown in FIG. 2, and executes print processing based on bitmapimage data generated by a controller 302. Reference numerals 303C, 303Y,303M, and 303K denote color discrepancy amount storage units whichrespectively store color discrepancy amounts for respective colors,i.e., cyan, yellow, magenta, and black. In this embodiment, discrepancyamounts in the sub-scan direction with respect to the ideal main scanline 201 based on the positions of the actual main scan line 202measured at the plurality of points, as described above using FIG. 10,are stored as information indicating the inclination and curvature ofthe main scan line 202 in the color discrepancy amount storage unit 303.

An example of data to be stored in this color discrepancy amount storageunit 303 (303C, 303Y, 303M, and 303K) is as shown in FIG. 12.

An operation for executing print processing by correcting image data tocancel the discrepancy amounts of the main scan lines stored in thecolor discrepancy amount storage unit 303 in the controller 302 will bedescribed below.

An image generating unit 304 generates raster image data, which allowsprint processing, based on print data received from an externalapparatus (not shown) such as a computer apparatus or the like, andoutputs that data as RGB data for respective dots. A color conversionunit 305 converts the RGB data into data on a CMYK space which can beprocessed by the printer engine 301. Each of halftone processing units309C to 309K reduces the number of bits of the input dot data using apredetermined halftone screen pattern to convert tone expression in adot unit into data of tone expression in area units based on thehalftone screen. The converted data is stored for each color in a bitmapmemory 306. The bitmap memory 306 temporarily stores the raster imagedata to be printed, and may comprise either a page memory that can storeimage data for one page or a band memory that can store data for aplurality of lines.

Reference numerals 307C, 307Y, 307M, and 307K denote color discrepancyamount arithmetic units which calculate correction amounts of colordiscrepancy corresponding to respective color data. These arithmeticunits calculate color discrepancy correction amounts in the sub-scandirection corresponding to coordinate information in the main scandirection designated by color discrepancy correcting units 308 (to bedescribed later) for respective dots on the basis of informationindicating the discrepancy amounts of the main scan lines stored in thecolor discrepancy amount storage units 303 corresponding to respectivecolors. The calculation results are output to the corresponding colordiscrepancy correcting units 308.

Let x (dots) be a coordinate of a given dot in the main scan direction,and y (lines) be a coordinate of that dot in the sub-scan direction.Also, let Δyi (dots) (i indicates a region) be the color discrepancycorrection amount in the sub-scan direction. In this case, arithmeticexpressions of the color discrepancy correction amounts Δyi in thesub-scan direction in respective regions based on FIG. 10 are as follows(note that the resolution in this case is 600 dpi).

Region 1: Δy1=x×(m1/L1)  (5)

Region 2: Δy2=m1×23.622+(x−L1×23.622)×((m2−m1)/(L2−L1))  (6)

Region 3: Δy3=m2×23.622+(x−L2×23.622)×((m3−m2)/(L3−L2))  (7)

The color discrepancy correcting units 308C, 308Y, 308M, and 308Krespectively correct color discrepancy due to the inclinations andcurvatures of the main scan lines. More specifically, these correctingunits adjust the output timings of bitmap data stored in the bitmapmemory 306 and adjust the exposure amounts for respective dots based onthe color discrepancy correction amounts calculated for respective dotsby the color discrepancy correction amount arithmetic units. In thismanner, any color discrepancy (registration errors) upon transferringtoner images of respective colors onto a transfer sheet can beprevented.

The color discrepancy correcting unit 308 (308C, 308Y, 308M, and 308K)according to this embodiment will be described below with reference tothe block diagram shown in FIG. 20. Note that a case of the cyan colordiscrepancy correcting unit 308C will be explained below. Since thecolor discrepancy correcting units for other colors have the samearrangements and operations, a description thereof will be omitted.

The color discrepancy correcting unit 308C comprises a coordinatecounter 801, coordinate converter 802, line buffer 803, and tonecorrector 804. The coordinate counter 801 outputs coordinate data in themain scan direction and sub-scan direction of a dot, that is to undergocolor discrepancy correction processing, to the coordinate converter802. At the same time, the coordinate counter 801 outputs coordinatedata in the main scan direction of that dot to the color discrepancycorrection amount arithmetic unit 307C and tone corrector 804. Thecoordinate converter 802 reads out line data to be processed from thebitmap memory 306 based on the coordinate data in the main scandirection and sub-scan direction from the coordinate counter 801. Thetone corrector 804 executes correction processing based on the integerpart of a discrepancy correction amount Δy based on this discrepancycorrection amount Δy obtained by the color discrepancy correction amountarithmetic unit 307C, i.e., reconstruction processing in the sub-scandirection in a dot unit. The tone corrector 804 also executes correctionprocessing based on the value of the decimal part of the discrepancycorrection amount Δy based on the coordinate data in the main scandirection from the coordinate counter 801 and the discrepancy correctionamount Δy, i.e., correction in less than a dot unit by adjusting theexposure ratios of neighboring dots in the sub-scan direction. The tonecorrector 804 uses the line buffer 803 to refer to neighboring dots inthe sub-scan direction.

The operation based on the above arrangement will be described below.

The coordinate converter 802 converts an address in the sub-scandirection of a coordinate value input from the coordinate counter 801,and reads out bitmap data of a corresponding line from the bitmap memory306. The line buffer 803 comprises a register 805 for storing dot datato be processed, and a FIFO buffer 806 for storing dot data for one lineof the previous line. The tone corrector 804 refers to dot data onneighboring lines in the sub-scan direction stored in the line buffer803 so as to generate correction data. The dot data stored in theregister 805 is output to the tone corrector 804 and is used to generatecorrection data for the next line. The tone corrector 804 receives acoordinate x in the main scan direction of the n-th line from thecoordinate counter 801. The tone corrector 804 also inputs x-th dot dataPn(x) of the n-th line from the register 805 and x-th dot data Pn−1(x)of the previous line from the FIFO buffer 806. The tone corrector 804executes the following arithmetic processing to generate correction dataP″n(x).

P′n(x)=Pn(x)×β(x)+Pn−1(x)×α(x)

A density confirmation unit 804 b receives the dot data Pn(x) to beprocessed to confirm the density of that dot. If the density of this dotPn(x) is lower than a predetermined density value (μ), the original dotdata Pn(x) is output as P″n(x); otherwise, the corrected dot data P′n(x)is selected and output. This selection is made by a selector 804 a. Inthis manner, the bitmap data whose color discrepancy amount less than adot unit in the sub-scan direction is corrected is output.

The dot data whose discrepancy amounts are corrected are converted intopulse-width modulated signals by PWM circuits 310C to 310K, and thesesignals are sent to corresponding exposure units 51C to 51K, thusdriving respective semiconductor lasers.

FIGS. 21A to 21C are conceptual views for explaining the operationexecuted when the coordinate converter 802 according to this embodimentcorrects a discrepancy amount based on the integer part of the colordiscrepancy correction amount Δy.

The coordinate converter 802 offsets a coordinate of dot data in thesub-scan direction (Y-direction), which is stored in the bitmap memory306, in accordance with the value of the integer part of the colordiscrepancy correction amount Δy calculated based on the colordiscrepancy information of the main scan lines approximated by straightlines, as indicated by reference numeral 600 in FIG. 21A. For example,as indicated by reference numeral 601 in FIG. 21B, when the coordinatein the sub-scan direction from the coordinate counter 801 is n (lines),letting x be a coordinate in the main scan direction, the colordiscrepancy correction amount Δy in region (1) in FIG. 21A satisfies0≦Δy<1. Hence, in this case, upon reconstructing data of the n-th line,dot data 610 of the n-th line is read out from the bitmap memory 306(offset=0). In region (2), the color discrepancy correction amount Δysatisfies 1≦Δy<2. Therefore, upon reconstructing the data of the n-thline, coordinate conversion processing for reading out data 611 of the(n+1)-th line offset by 1 as the number of sub-scan lines is executed.Likewise, in region (3), since the color discrepancy correction amountΔy satisfies 2≦Δy<3, coordinate conversion processing for reading outdata 612 of the (n+2)-th line is executed. Also, similarly, coordinateconversion processing for reading out data of the (n+3)-th line forregion (4) and that for reading out data of the (n+4)-th line for region(5) are executed.

With the above method, reconstruction processing in a line unit in thesub-scan direction, i.e., in a dot unit, is executed in accordance withthe value of the integer part of the discrepancy correction amount. Notethat reference numeral 602 in FIG. 21C denotes an exposed image when animage is exposed on the photosensitive drum based on the data 601 whichhas undergone the color discrepancy correction in a dot unit by thecoordinate converter 802.

FIGS. 22A to 22F are conceptual views for explaining the operationcontents of color discrepancy correction in less than a dot unitperformed by the tone corrector 804, i.e., those for correcting thediscrepancy amount of the decimal part of the color discrepancycorrection amount Δy. The discrepancy amount of the decimal part iscorrected by adjusting the exposure ratios of neighboring dots in thesub-scan direction.

FIG. 22A shows an image of a main scan line having a right upwardinclination. FIG. 22B shows a bitmap image before discrepancycorrection, i.e., a bitmap image of a straight line which is horizontalin the main scan direction. FIG. 22C shows a correction image obtainedby correcting the bitmap image in FIG. 225 to cancel color discrepancydue to the inclination of the main scan line shown in FIG. 22A.

In this embodiment, in order to realize the correction image shown inFIG. 22C, the exposure amounts of dots on neighboring lines located inthe sub-scan direction are adjusted.

FIG. 22D shows the relationship between the color discrepancy correctionamount Δy and correction coefficients used to attain tone correction. kis the integer part (obtained by truncating the decimal part) of thecolor discrepancy correction amount Δy, i.e., a correction amount in adot unit described using FIGS. 21A to 21C. β and α are correctioncoefficients, which are used to perform correction in less than a dotunit (a size smaller than one dot size) in the sub-scan direction, andrepresent distribution ratios of an exposure amount of neighboring dotsin the sub-scan direction. These correction coefficients arerespectively given by

α=Δy−k

β=1−α

where α is the distribution ratio of a dot to the (n+k)-th line, and βis that of a dot to the (n+k−1)-th line. That is, when k=0, α is thedistribution ratio of a dot to the n-th line, and β is that of a dot tothe (n−1)-th line. Also, when k=1, α is the distribution ratio of a dotto the (n+1)-th line, and β is that of a dot to the n-th line.

A description will be given with reference to FIGS. 22C and 22D. A dot700 is formed on the (n−1)-th line immediately before the n-th linewhere it is originally located, since k=0, α=0, and β=1 in FIG. 22D. Adot 701 is formed ¾ on the (n−1)-th line immediately before the n-thline, and is formed ¼ on the current line (n), since α=0.25 and β=0.75in FIG. 22D. Likewise, a dot 702 is formed ½ on the (n−1)-th lineimmediately before the n-th line, and is formed ½ on the current line(n), since α=0.5 and β=0.5 in FIG. 22D. Also, a dot 703 is formed ¼ onthe (n−1)-th line immediately before the n-th line, and is formed ¾ onthe current line (n). Note that a dot 704 is formed at the position ofthe n-th line where it is originally located, since k=1, α=0, and β=1.For dots 705 to 707, since k=1, a represents the distribution ratio of adot to the (n+1)-th line, and β represents that of a dot to the n-thline. Furthermore, for a dot 708, since k=2, α represents thedistribution ratio of a dot to the (n+2)-th line, and β represents thatof a dot to the (n+1)-th line.

FIG. 22E shows pulse signals based on the bitmap image in which theexposure ratios of the neighboring dots in the sub-scan direction areadjusted in accordance with the correction coefficients in FIG. 22D.FIG. 22E illustrates each pulse signal in the form of a pulse-widthmodulated signal corresponding to each dot data.

FIG. 22F shows an image developed on the photosensitive drum whenexposure is made by a laser based on the pulse widths that haveundergone tone correction, as shown in FIG. 22E. In this manner, theinclination of the main scan line is canceled, and a horizontal straightline is formed.

In the above description, correction processing implemented by hardwarehas been explained. When the controller 302 comprises a CPU, as shown inFIG. 16, processing can also be implemented by software as in the abovethird embodiment.

FIGS. 23 and 24 are flowcharts for explaining image forming processingto be executed by the CPU 1000 when the controller 302 according to thisembodiment has the arrangement shown in FIG. 16. A program thatimplements this processing is stored in the ROM 1001, and is executedunder the control of the CPU 1000.

In step S1, the color discrepancy amounts for respective colors storedin the color discrepancy amount storage units 303C to 303K of theprinter engine 301 are read out, and are stored in the areas 1010 on theRAM 1002. In step S2, print data is input and undergoes processing suchas color conversion and the like. After that, the print data isconverted into cyan, yellow, magenta, and black bitmap image data eachfor one page, and the converted data are stored in the bitmap memory306. In step S3, a variable n used to count the number of lines is resetto “1”, and a variable x used to count the dot position (x coordinate)is reset to “0”. Note that both these variables are stored in the RAM1002.

In step S4, the x-th dot data of the n-th line is read out from the cyanbitmap data. In step S5, a region which includes that dot (for example,one of regions 1 to 3 in FIG. 10) is determined. In step S6, acorrection amount Δy in the sub-scan direction, which forms that dot iscalculated based on the region determined in step S5 and the dotposition (x). This value can be calculated using one of equations (5) to(7) above. It is checked in step S7 if the integer part of thecorrection amount Δy calculated in step S6 is “0”. If the integer partis “0”, since correction in a line unit is not required, the flow jumpsto step S10 a; otherwise, the flow advances to step S8 to check if theinteger part is positive or negative. If the integer part is positive,the flow advances to step S9 to acquire the x-th dot data of the(n+s)-th line and specify it as the dot data of the current line (seeFIGS. 21A to 21C). On the other hand, if it is determined in step S8that the integer part is negative, the flow advances to step S10 to thex-th dot data of the (n−s)-th line and specify it as the dot data of thecurrent line (see FIGS. 21A to 21C). Note that s indicates the absolutevalue of that integer part. After step S9, or S10, the flow advances tostep S10 a. It is checked in step S10 a if the density of the dot data(multi-valued data) to be processed is lower than a predetermineddensity (μ). This corresponds to the arrangement of the toneconfirmation unit 804 b shown in FIG. 20. If the density value is lowerthan the threshold (μ), it is determined that the aforementionedcorrection using the coefficients α and β is not required, and the flowjumps to step S12. On the other hand, if the density value is higherthan the threshold (μ), since that dot becomes conspicuous if it isformed, it is determined that the aforementioned discrepancy correctionis required, and the flow advances to step S11.

In step S11, processing for the numerical value of the decimal part ofthe correction amount Δy is executed in turn. According to the numericalvalue of the decimal part, the distribution of the x-th dot data on thecurrent line (n-th line) and the (n+1)-th or (n−1)-th line isdetermined. As has been described above with reference to FIGS. 22A to22F, dot data are exchanged or replaced between those on neighboringlines in accordance with the numerical value of the decimal part. Inthis manner, after the x-th dot data on the current line (n-th line) isupdated, the bitmap data is updated in step S12. In step S13, thevariable x is incremented by +1. It is checked in step S14 if the valueof the variable x becomes larger than the total number of dots of oneline. If the value of the variable x is smaller than the total number ofdots, the flow returns to step S4 to repeat the aforementionedprocessing.

If it is determined in step S14 that the value of the variable x becomeslarger than the total number of dots of one line, the flow advances tostep S15 to increment the variable n used to count the number of linesby +1. It is then checked in step S16 if the value of this variable nexceeds the number of lines of one page. If the value of this variable ndoes not exceed the number of lines of one page, the flow advances tostep S17 to reset the variable x to “0”. The flow then returns to stepS4 to repeat the aforementioned processing. On the other hand, If it isdetermined in step S16 that the value of the variable n exceeds thenumber of lines of one page, the flow advances to step S18. It ischecked in step S18 if processing for the cyan, yellow, magenta, andblack bitmap data is complete. If the processing is not complete yet,the flow returns to step S3 to repeat the aforementioned processing;otherwise, the flow advances to step S19 to start image formingprocessing.

In step S19, a transfer sheet is picked up from the cassette 53 andbegins to be fed. While the transfer sheet is fed placed on the feedingbelt 10, dot data is PWM-modulated. Toner images are formed in turn inthe order of cyan, yellow, magenta, and black (step S20), and they aretransferred in turn on the fed transfer sheet (step S21). Uponcompletion of transfer, the toner images are fixed on the transfer sheetin step S22. Upon completion of fixing, the fixed transfer sheet isdischarged in step S23.

Note that the aforementioned threshold (μ) may be set for each color orbeam. For example, in case of a color such as yellow whose densitydifference is difficult to visually determine, the threshold μ is set tobe larger than those of other colors. In this way, since the ratio ofexecution of tone correction is reduced compared to other colors, moirécan be eliminated.

As described above, with the color image forming apparatus according tothis embodiment, both color discrepancy in a dot unit and that in anamount less than a dot unit can be corrected based on the colordiscrepancy amounts on respective photosensitive drums. In this way,color discrepancy of respective color images due to the inclinations,curvatures, and the like of scan lines that scan and expose therespective photosensitive drums can be prevented, thus obtaining ahigh-quality color image.

Fifth Embodiment

The method disclosed in patent reference 2 above further suffers thefollowing problems. That is, when image data is output while executingcoordinate conversion, comprehensive misalignments can be coped with,but stripe-like spots are generated due to quantization errors. When thecoordinate value of the dot position after coordinate conversionincludes a fraction below the decimal point, with an arrangement whichforms a dot around a location where that point is ideally located byreducing the toner amount, stripe-like spots due to quantization errorscan be prevented to some extent. However, in order to obtain ahigh-quality image, a toner discharge amount must be finely controlled.In order to finely control the toner discharge amount, the number ofbits to be assigned per pixel of image data must be increased. For thisreason, this arrangement requires a large-capacity memory that storesimage data, resulting in an increase in manufacturing cost of theapparatus. Note that this problem is also posed when a monochrome imageis formed. The fifth embodiment solves such drawbacks.

The arrangement of the color image forming apparatus shown in FIG. 2will be quoted in this embodiment.

FIG. 25 is a block diagram for explaining color discrepancy correctionprocessing for correcting color discrepancy that occurs due to theinclination and curvature of the main scan line in this embodiment.

Referring to FIG. 25, reference numeral 401 denotes a printer engine,which executes print processing based on image bitmap informationgenerated by a controller 402, i.e., based on drive signals input fromPWM units 410 (to be described later).

Reference numerals 403C, 403Y, 403M, and 403K (to be referred to as 403together hereinafter) denote color discrepancy amount storage units forbasic colors. These color discrepancy amount storage units respectivelystore information associated with color discrepancy of the main scanlines for respective regions mentioned above. In this embodiment, eachcolor discrepancy amount storage unit 403 stores the difference betweenthe actual main scan line 202 and ideal main scan line 201 asinformation indicating the inclination and curvature of the main scanline, as described above using FIG. 10.

FIG. 26 shows an example of data to be stored in the color discrepancyamount storage unit 403. Regions 1 to 3 in FIG. 26 correspond to thosein FIG. 10. For example, the widths of regions 1 to 3 in FIG. 26respectively correspond to those of regions 1 to 3 in FIG. 10, i.e., thex-coordinate difference of (Pa, Pb), that of (Pb, Pc), and that of (Pc,Pd). The discrepancy amounts of regions 1 to 3 in FIG. 26 correspond tothose of regions 1 to 3 in FIG. 10, i.e., the y-coordinate difference of(Pa, Pb), that of (Pb, Pc), and that of (Pc, Pd).

In this embodiment, each color discrepancy amount storage unit 403stores the discrepancy amount between the ideal main scan line andactual main scan line as information associated with color discrepancy.However, the present invention is not limited to such specificinformation as long as the characteristics or the like of theinclination and curvature of the actual main scan line can be derived(e.g., the inclination of the actual main scan line, coordinates of theend points, and the like). As information to be stored in each colordiscrepancy amount storage unit 403, the above discrepancy amount may bemeasured in the manufacturing process of the apparatus, and may bepre-stored as information unique to the apparatus. Alternatively, adetection mechanism that detects the discrepancy amount may be preparedto form a predetermined pattern used to measure discrepancy for eachphotosensitive drum 14 of a basic color, and the discrepancy amountdetected by the detection mechanism may be stored.

An operation for executing print processing by correcting image data tocancel the discrepancy amounts of the main scan lines stored in thecolor discrepancy amount storage units 403 in the controller 402 will bedescribed below.

An image generating unit 404 generates raster image data, which allowsprint processing, based on print data received from an externalapparatus (not shown) such as a computer apparatus or the like, andoutputs that data as RGB (Red, Green, Blue) data for respective dots.Reference numeral 405 denotes a color conversion unit which converts theRGB data output from the image generating unit 404 into data on a CMYKspace which can be processed by the controller 402, and stores theconverted data in a bitmap memory (image memory) 406 to be describedbelow for respective basic colors. The bitmap memory 406 temporarilystores the raster image data to be printed, and may comprise either apage memory that can store image data for one page or a band memory thatcan store data for a plurality of lines.

Reference numerals 307C, 307Y, 307M, and 307K denote color discrepancycorrection amount arithmetic units. These arithmetic units calculatecolor discrepancy correction amounts in the sub-scan directioncorresponding to coordinate information in the main scan directiondesignated by color discrepancy correcting units 408 (to be describedlater) for respective dots on the basis of information indicating thediscrepancy amounts of the main scan lines stored in the colordiscrepancy amount storage units 403 corresponding to respective colors.The calculation results are output to the corresponding colordiscrepancy correcting units 408.

Each color discrepancy correction amount arithmetic unit 307 calculatesa color discrepancy correction amount in the sub-scan direction byexecuting the following arithmetic operations. That is, letting x (dots)be coordinate data in the main scan direction, and Δy (dots) be a colordiscrepancy correction amount in the sub-scan direction, arithmeticexpressions of respective regions based on FIGS. 10 and 26 are asfollows, Assume that the following arithmetic expressions are given whenthe print density is 600 dpi.

Region 1: Δy1=x*(m1/L1)

Region 2: Δy2=m1*23.622+(x−L1*23.622)*((m2−m1)/(L2−L1))

Region 3: Δy3=m2*23.622+(x−L2*23.622)*((m3−m2)/(L3−L2))

As shown in FIG. 10, L1, L2, and L3 are distances (unit: mm) in the mainscan direction from the print start position to the left ends of regions1, 2, and 3. Also, m1, m2, and m3 are discrepancy amounts between theideal main scan line 201 and actual main scan line 202 at the left endsof regions 1, 2, and 3.

Referring back to FIG. 25, reference numerals 408C, 408Y, 408M, and 408K(to be referred to as 408 together hereinafter) denote color discrepancyamount correcting units. This correcting unit adjusts the output timingsof bitmap data stored in the bitmap memory 406 and adjusts the exposureamounts for respective pixels based on the color discrepancy correctionamounts calculated for respective dots by the corresponding colordiscrepancy correction amount arithmetic unit 307, so as to correctcolor discrepancy due to the inclination and curvature of the main scanline. In this way, color discrepancy upon transferring toner images ofrespective basic colors onto a transfer sheet can be prevented.

The color discrepancy correcting unit 408 will be described below withreference to FIG. 27. FIG. 27 is a block diagram showing the arrangementof the color discrepancy amount correcting unit 408.

As shown in FIG. 27, the color discrepancy correcting unit 408 comprisesa coordinate counter 801, coordinate converter 802, line buffer 803, andtone corrector 804. The coordinate counter 801 outputs coordinate datain the main scan direction and sub-scan direction of a dot, that is toundergo color discrepancy correction processing, to the coordinateconverter 802. At the same time, the coordinate counter 801 outputscoordinate data in the main scan direction of that dot to the colordiscrepancy correction amount arithmetic unit 307 and tone corrector804. The coordinate converter 802 as conversion means executescorrection processing of the integer part of the correction amount Δybased on the coordinate data in the main scan direction and sub-scandirection from the coordinate counter 801 and the correction amount Δyobtained from the color discrepancy correction amount arithmetic unit307. That is, the coordinate converter 802 executes reconstructionprocessing in the sub-scan direction in the pixel unit. The tonecorrector 804 as acquisition means executes correction processing of thedecimal part of Δy based on the coordinate data in the main scandirection from the coordinate counter 801 and the correction amount Δy,i.e., it performs correction in less than the pixel unit by adjustingthe exposure ratios of neighboring dots in the sub-scan direction. Thetone corrector 804 uses the line buffer (holding means) 803 to refer toneighboring dots in the sub-scan direction.

In this manner, the color discrepancy amount correcting unit 408comprises the coordinate converter 802 which executes correctionprocessing of the integer part of the correction amount Δy obtained fromthe color discrepancy correction amount arithmetic unit 307, i.e.,reconstruction processing in the sub-scan direction in the pixel unit.Furthermore, the unit 408 also comprises the tone corrector 804 whichexecutes correction processing of the decimal part of Δy, i.e., performscorrection in less than the pixel unit by adjusting the exposure ratiosof neighboring dots in the sub-scan direction. The tone corrector 804uses the line buffer to refer to neighboring dots in the sub-scandirection.

The processing of the coordinate converter 802 of the color discrepancyamount correcting unit 408 will be described below with reference toFIGS. 28A to 28C. FIGS. 28A to 28C are views illustrating the operationcontents of the coordinate converter 802 for correcting the discrepancyamount of the integer part of the color discrepancy correction amountΔy.

The coordinate converter 802 offsets a coordinate of image data in thesub-scan direction (Y-direction), which is stored in the bitmap memory406, in accordance with the value of the integer part of the colordiscrepancy correction amount Δy calculated based on the colordiscrepancy information of the main scan lines approximated by straightlines, as shown in FIG. 28A. Referring to FIG. 28B, let X be acoordinate position in the main scan direction. When the coordinateposition in the sub-scan direction from the coordinate counter 801 is n,at the X-coordinate in the main scan direction, the color discrepancycorrection amount Δy in region (A) satisfies 0≦Δy<1, and the offsetamount is 0. Hence, upon reconstructing data of the n-th line, data ofthe n-th line is read out from the bitmap memory. In region (B), thecolor discrepancy correction amount Δy satisfies 1≦Δy<2. Therefore, uponreconstructing the data of the n-th line, coordinate conversionprocessing for reading out an image bitmap at a position offset by thenumber of sub-scan lines=1, i.e., data of the (n+1)-th line from thebitmap memory is executed. Likewise, coordinate conversion processingfor reading out data of the (n+2)-th line for region (C) and that forreading out data of the (n+3)-th line for region (D) are executed. Withthe above method, reconstruction processing in the pixel unit in thesub-scan direction is executed.

FIG. 28C shows an exposed image when image data which has undergone thecolor discrepancy correction in the pixel unit by the coordinateconverter 802 is exposed on the photosensitive drum 14. Even when themain scan direction obliquely misaligns upon image formation, since theaforementioned reconstruction processing in the sub-scan direction isexecuted, an image of a horizontal straight line can be formed on atransfer sheet in a form approximate to that of an original image.

The processing of the tone corrector 804 for performing correction inless than the pixel unit by adjusting the exposure ratios of neighboringdots in the sub-scan direction will be described below with reference toFIGS. 29A to 29F. FIGS. 29A to 29F are views illustrating the operationcontents of color discrepancy correction in less than the pixel unitexecuted by the tone corrector 804, i.e., those for correcting thediscrepancy amount of the decimal part of the color discrepancycorrection amount Δy. The discrepancy amount of the decimal part can becorrected by adjusting the exposure ratios of neighboring dots in thesub-scan direction.

FIG. 29A shows an image of a main scan line having a right upwardinclination. FIG. 29B shows a bitmap image before tone correction, i.e.,a bitmap image of a horizontal straight line of an original image. FIG.29C shows a correction bitmap image after the bitmap image shown in FIG.29B has undergone tone correction so as to cancel color discrepancy dueto the inclination of the main scan line shown in FIG. 29A. Thecorrection bitmap image shown in FIG. 29C is ideal when the inclinationdiscrepancy amount is given by, e.g., FIG. 29A. The tone corrector 804forms an image approximate to the correction bitmap image by adjustingthe exposure amounts of regular grid points and the toner dischargeamounts, so as to form an image approximate to the correction bitmapimage shown in FIG. 29C.

In order to realize the correction image shown in FIG. 29C, the exposureamounts of neighboring dots in the sub-scan direction are adjusted. FIG.29D shows the relationship between the color discrepancy correctionamount Δy and correction coefficients used to attain tone correction. kis the integer part (obtained by truncating the decimal part) of thecolor discrepancy correction amount Δy, i.e., a correction amount in thesub-scan direction in the pixel unit. β and α are correctioncoefficients, which are used to perform correction in less than thepixel unit in the sub-scan direction, and represent distribution ratiosof an exposure amount of neighboring dots in the sub-scan directionbased on information of the decimal part of the color discrepancycorrection amount Δy. These correction coefficients are respectivelycalculated by:

α=Δy−k

β=1−α

where α is the distribution ratio to the dot to be scanned, and β isthat to the trailing dot.

FIG. 29E illustrates a bitmap image which has undergone tone correctionto adjust the exposure ratios of neighboring dots in the sub-scandirection based on the correction coefficients α and β in FIG. 29D. FIG.29F shows an exposed image of the bitmap image, which has undergone tonecorrection, on the photosensitive drum 14, i.e., a state wherein theinclination of the main scan line is canceled by the bitmap image whichhas undergone tone correction, and a horizontal straight line is formed.

The tone correction processing will be described below with referenceagain to FIG. 27. The coordinate converter 802 reconstructs image datainput from the bitmap memory 406 to correct color discrepancy amounts inthe pixel unit based on the correction amounts acquired from the colordiscrepancy correction amount arithmetic unit 307. More specifically,the coordinate converter 802 executes processing for converting thecoordinate of the read address of the bitmap memory 406 based on thecorrection amount acquired from the color discrepancy correction amountarithmetic unit 307, and reading out image data based on the convertedaddress information. In this manner, for example, the coordinateconverter 802 acquires information of pixels corresponding to dots whichline up in the main scan direction (corresponding to, e.g., the rightdirection in FIG. 28B) on an oblique basis system shown In, e.g., FIGS.28C and 29F in turn from the bitmap memory 406. The coordinate converter802 transfers this reconstructed image data to the line buffer 803. Thecoordinate converter 802 calculates the values α and β by theaforementioned arithmetic operations from Δy acquired by the colordiscrepancy correction amount arithmetic unit 307, and outputs them tothe tone corrector 804.

Note that the color discrepancy amount correcting unit 408 may receivethe value of the correction amount Δy from the color discrepancycorrection amount arithmetic unit 307 every time it outputs pixelinformation for one dot to a halftone processing unit 409.Alternatively, the color discrepancy amount correcting unit 408 mayreceive values of the correction amounts Δy for one line from the colordiscrepancy correction amount arithmetic unit 307 prior to theprocessing, and may proceed with processing based on these values.

In the above arrangement, the coordinate converter 802 calculates α, β,and the like from the correction amount Δy. However, the colordiscrepancy correction amount arithmetic unit 307 may calculate α, β,and the like from the correction amount Δy stored in the colordiscrepancy amount storage unit 403, and may output them in response toa request from the components of the color discrepancy amount correctingunit 408. In this case, the color discrepancy amount correcting unit 408may acquire α and β for one line in advance from the color discrepancycorrection amount arithmetic unit 307 prior to the processing, or mayacquire them every time it processes pixel information for one dot.

The line buffer 803 is a storage device which temporarily buffers imagedata for a predetermined line since the tone corrector 804 must refer toneighboring pixel values in the sub-scan direction upon generatingcorrection data. In this embodiment, the data, size to be buffered isfor one line of image data for the sake of simplicity. However, data fortwo or more lines may be buffered.

The line buffer 803 comprises a FIFO (First In First Out) buffer 806which stores data for one line of the previous line, and a register 805which holds pixel data of a coordinate that is to undergo the tonecorrection processing. The pixel data stored in the register 805 isoutput to the tone corrector 804, and is also stored in the FIFO buffer806 so as to be used in generation of correction data for the next line.

Let x (dots) be a coordinate in the main scan direction, Pn(x) be pixeldata input from the register 805, and Pn−1(x) be pixel data input fromthe FIFO buffer 806. At this time, the tone corrector 804 executes thefollowing arithmetic processing to generate correction data.

P′n(x)=Pn(x)*β(x)+Pn−1(x)*α(x)

Note that the values α and β are acquired from the coordinate converter802, as described above. The tone corrector 804 outputs the value ofP′n(x) calculated by the above arithmetic processing to the halftoneprocessing unit 409 as an image bitmap whose color discrepancy amountless than the pixel unit in the sub-scan direction is corrected.

Upon reception of image data that has undergone the color discrepancycorrection from the tone corrector 804 (color discrepancy correctingunit 408), each of halftone processing units 409C, 409M, 409Y, and 409K(to be referred to as 409 together hereinafter) executes halftoneprocessing using a predetermined halftone pattern. The processed imagedata is output to each of PWM (Pulse Wide (or Width) Modulation) units410C, 410M, 410Y, and 410K (to be referred to as 410 togetherhereinafter).

Upon reception of the image data that has undergone the halftoneprocessing, each PWM unit applies pulse width modulation processing tothat image data, and outputs the processed data to the printer engine401 as a drive signal. The printer engine 401 executes exposureprocessing on each photosensitive drum 14, development processing,transfer processing onto a transfer sheet, and the like based on thereceived drive signal.

In this embodiment, the tone corrector 804 performs bit expansion ofinput image data in addition to the aforementioned processing, andoutputs the bit-expanded image data to the halftone processing unit 409and PWM unit 410 to allow detailed image formation. The processing andeffect of this tone corrector 804 will be described in detail below withreference to FIGS. 30A to 30H. FIGS. 30A to 30H are views for explainingthe processing for assigning many bits to the number of bits of dataoutput from the bitmap memory upon performing tone correction so as toobtain a higher-quality image.

In the example shown in FIGS. 30A to 30H, assume that each pixel ofimage data input to the tone corrector is expressed by 2 bits. FIG. 30Ashows an image of a main scan line having a right upward inclination.FIG. 30B shows a bitmap image of a horizontal straight line before tonecorrection, and FIG. 30C shows a correction bitmap image of FIG. 30B tocancel color discrepancy due to the inclination of the main scan line inFIG. 30A. In order to realize the correction bitmap image in FIG. 30C,the exposure amounts of neighboring dots in the sub-scan direction areadjusted. FIG. 30D shows a list of Δy and corresponding values of k, α,and β. The values of α and β are calculated by the aforementionedequations (α=Δy−k and β=1−α).

The tone corrector 804 performs color discrepancy correction in lessthan the pixel unit based on the correction coefficients shown in, e.g.,FIG. 30D. Furthermore, the tone corrector 804 executes processing forexpanding the bit width of each pixel. The effect of the processing forexpanding the bit width will be described below with reference to FIGS.308 to 30H.

FIG. 30E shows a bitmap image on the photosensitive drum 14 when eachpixel after tone correction is expressed by 2 bits, and FIG. 30F shows abitmap image when each pixel after tone correction is expressed by 4bits.

The correction coefficients shown in FIG. 30D are divided into a totalof 10 gray levels. However, since the number of bits (bit width) in FIG.30E is only 2 bits, each pixel value can only express up to four graylevels. Hence, the calculated correction value must be rounded to reducethe number of gray levels to four to express halftone. In this case, anexposed image on the photosensitive drum 14 is as shown in FIG. 30G.

By contrast, since the number of bits is expanded to 4 bits in FIG. 30F,each pixel value can express up to 16 gray levels. For this reason, theround error of the correction coefficients calculated in FIG. 30D can bereduced. In this case, an exposed image on the photosensitive drum 14 isas shown in FIG. 30H. As can be seen from comparison with FIG. 30G of 2bits, an accurate, high-quality image can be obtained.

Assume that information about the expansion range of the number of bitsper pixel is stored in a predetermined storage device, and the tonecorrector 804 refers to this information upon bit expansion and controlsthe bit expansion processing based on that information. In the abovedescription, input data is expressed by 2 bits. However, when the numberof bits to be assigned to one pixel is increased using the samearrangement, the same effect can be obtained.

As described above, since the tone corrector 804 expands the bit width(the number of bits) per pixel and then outputs the data to the halftoneprocessing unit, an accurate, detailed image can be formed even in anenvironment in which respective image forming units suffer colordiscrepancy. In the above arrangement, since the tone corrector 804executes processing for expanding the bit width of each pixel, eachpixel of image data to be input to the color discrepancy correcting unitmay have a normal bit width. Therefore, according to the abovearrangement, an accurate, detailed image can be formed withoutincreasing the capacity of storage devices such as the bitmap memory406, line buffer 803, and the like.

In the above arrangement, information about the expansion range of thenumber of bits per pixel is stored in the predetermined storage device.However, the embodiment of the present invention is not limited to suchspecific arrangement. For example, an instruction input device whichserves as a user interface which can be operated by the user may beprovided, an instruction input indicating the number of bits to beexpanded or the like, and the bit expansion per pixel may be controlledbased on this instruction input. With this arrangement, the user caneasily set details and accuracy of image formation in accordance withthe use application and purpose.

Sixth Embodiment

Conventionally, various methods of forming an electrostatic latent imageby irradiating a photosensitive member with a light beam, and forming avisible image on a print medium by developing the latent image withtoner in an image forming apparatus using an electrophotographic methodhave been proposed. Upon forming an image by such electrophotographicmethod, an image formed on the photosensitive member suffers distortiondue to errors of the positional precision and diameter of thephotosensitive member, and an positional precision error of an opticalsystem. As a method of correcting such distortion in an image, a methodof mechanically correcting the optical path of the optical system, and amethod of correcting such distortion by applying image processing suchas coordinate conversion or the like to an image may be used. However,these methods pose the following problems.

In order to correct the optical path of the optical system, a correctionoptical system including a light source and f-θ lens, a mirror in theoptical path, and the like must be mechanically moved to adjust theposition of the test toner image. However, for this purpose,high-precision movable members are required, resulting in high cost ofthe apparatus. Furthermore, since it takes much time until correction iscompleted, it is nearly impossible to frequently perform correction.However, the optical path length difference may change along with anelapse of time due to temperature rise of mechanical components. In suchcase, it becomes difficult to prevent any misalignment by correcting theoptical path of the optical system.

In contrast to the aforementioned mechanical correction, attempts tocancel misalignment by converting image data are described in JapanesePatent Application Laid-Open No. 3-85236 (patent reference 3) and abovepatent reference 2 (Japanese Patent Application Laid-Open No. 8-85237).

Patent reference 3 discloses an arrangement which automatically convertsthe output coordinate position of image data for each color into thatwhose registration error is corrected, and corrects the position of eachlight beam based on the converted image data for each color. Forexample, in a system in which an image shown in FIG. 31A is distorted,as shown in FIG. 31B, image data which has undergone position correctionfor each dot, as shown in FIG. 31C, is generated, and is printed tocancel the distortion. However, upon printing image data shown in FIG.31C, an image with steps is formed, as shown in FIG. 31D, thusdeteriorating image quality.

Patent reference 2 discloses an arrangement which automatically convertsthe output coordinate position of image data for each color into thatwhose registration error is corrected, and corrects the position of alight beam modulated based on the converted image data for each color inan amount smaller than the minimum dot unit of the color signal.However, with the method of patent reference 2, when the outputcoordinate position of image data for each color, which has undergonehalftone processing, is corrected, the reproducibility of halftone dotsof a halftone image deteriorates. As a result, color inconsistency mayoccur and moiré may become obvious.

FIGS. 32A to 32C show an example. An input image shown in FIG. 32A has aconstant density value. An image after color discrepancy correctionshown in FIG. 32B is obtained by applying predetermined misalignmentcorrection to the input image. In general, the relationship between theimage density value and toner density corresponding to the image densityvalue is not linear, as indicated by “toner density” shown in FIG. 32C.For this reason, when the image after color discrepancy correction inFIG. 32B is printed, although the input image in FIG. 32A has a constantdensity value, an image whose density value is not constant is printed.When such non-uniform density values are periodically repeated, moirébecomes obvious, and a high-quality color image cannot be obtained.Since correction is made by calculating correction amounts less than onepixel, the arrangement becomes complicated, resulting in high cost.

Japanese Patent Application Laid-Open No. 9-90695 (patent reference 4)discloses an arrangement which skips image correction in case of amonochrome image. However, if no correction is applied, an image whichis distorted, as shown in FIG. 31B, is not corrected, and a high-qualityimage cannot be obtained.

Therefore, the sixth embodiment has as its object to provide an imageforming apparatus which effectively corrects any image distortion by asimple arrangement, and can acquire a high-quality image with low cost.

The arrangement of an image forming apparatus according to thisembodiment will be described below with reference to FIG. 33A. FIG. 33Ais a sectional view showing the internal structure of the image formingapparatus (laser beam printer) according to this embodiment.

Referring to FIG. 33A, an image forming apparatus 100 receives andstores print information (text codes, etc.), form information, macrocommands, and the like, supplied from an externally connected hostcomputer (not shown). After that, the apparatus 100 generates acorresponding text pattern, form pattern, or the like according to thereceived information, and forms a visible image on a print sheet as aprint medium. Reference numeral 300 denotes a control panel on whichoperation switches, LED indicators, and the like are arranged; and 101,a printer controller which controls the overall image forming apparatus100 and interprets text information and the like supplied from the hostcomputer. The printer controller 101 mainly converts text informationinto a video signal of a text pattern, and outputs the converted videosignal to a laser driver 102.

The laser driver 102 is a circuit for driving a semiconductor laser 31,and turns on/off a laser beam 104 emitted by the semiconductor laser 31in accordance with the input video signal. This laser beam 104 isscanned in the right-and-left directions by a rotary polygonal mirror(polygon mirror) 32 to expose the surface of a photosensitive drum 33.As a result, an electrostatic latent image of the text pattern is formedon the photosensitive drum 33. This latent image is developed by adeveloping unit 107 arranged around the photosensitive drum 33, and isthen transferred onto a print sheet. This print sheet uses a cut sheet,which is stored in a paper cassette 108 attached to the LBP (imageforming apparatus) 100. The cut sheet is picked up into the apparatus bya paper feed roller 109 and guide rollers 110 and 111, and is fed to thephotosensitive drum 33.

FIG. 33B is a schematic view for explaining the arrangement of anoptical system of the image forming apparatus 100 according to the sixthembodiment. Referring to FIG. 33B, the laser unit 31 turns on and off inaccordance with a PWM signal generated by the printer controller 101 andlaser driver 102. The polygon mirror 32 rotates about a rotational axis34. A laser beam from the laser unit 31 is scanned in the main scandirection (direction of a rotational axis 35) upon rotation of thepolygon mirror 32 to expose the surface of the photosensitive drum 33.The photosensitive drum 33 rotates about the rotational axis 35, and anelectrostatic latent image corresponding to an image to be printed isformed on the photosensitive drum by exposure.

In such optical system, the positional precisions of the laser unit 31,polygon mirror 32, photosensitive drum 33, the rotational axis 34 of thepolygonal mirror, and the rotational axis 35 of the photosensitive drum33 are important. However, the mechanical positional precisions havelimits, and a main scan line 37 of the laser beam on the photosensitivedrum 33 has an inclination with respect to an ideal main scan line 36parallel to the rotational axis 35 due to misalignment of thesecomponents. An arrangement for reducing an image distortion caused bysuch inclination of the main scan line will be described below.

FIG. 34 is a block diagram for explaining the control arrangement forimplementing correction processing which is executed in the sixthembodiment and corrects the inclination of the main scan line.

Referring to FIG. 34, a printer engine 1401 executes actual printprocessing based on image bitmap information generated by the printercontroller 101 (the laser driver 102, semiconductor laser 31, polygonmirror 32, photosensitive drum 33, paper feeding system, and the like inFIG. 33A). A horizontal sync signal generator 1404 outputs a signal forsynchronizing a write start position in the main scan direction to theprinter controller 101. A misalignment amount storage unit 1403 measuresand stores information (angle θ) indicating the inclination of theactual main scan line 37 with respect to the ideal main scan line (35)shown in FIG. 33B.

In this embodiment, the angle θ is stored as information indicating theinclination of the main scan line. However, the present invention is notlimited to this as long as the inclination of the actual main scan lineis identifiable information. For example, (1) the fact that when aposition advances x in the main scan line direction, it deviates y inthe sub-scan direction (≈tan θ), (2) the fact that when a positionadvances x in the main scan line direction, it deviates 1 in thesub-scan direction (substantially the same as (1)), (3) a product(Ly·sin θ) of a distance (Ly) between the main scan lines and theinclination, or the like may be held as information. As the informationstored in the misalignment amount storage unit 1403, the misalignmentamount (θ) is measured in the manufacturing processing of this imageforming apparatus 100, and is pre-stored as information unique to theapparatus. Alternatively, the image forming apparatus 100 may comprise aknown detection mechanism for detecting the misalignment amount. In thiscase, a predetermined pattern used to measure the misalignment amount isformed on the photosensitive drum 33, and the misalignment amountdetected by the detection mechanism is stored in the misalignment amountstorage unit 1403. If the arrangement comprising the detection mechanismis adopted, a change in characteristic of the image forming apparatus(exposure unit 1411) over time can be coped with.

The control for executing print processing by correcting the outputposition in the main scan direction to correct the misalignment amountof the main scan line stored in the misalignment amount storage unit1403 in the printer controller 101 will be described below.

An image generating unit 1405 generates raster image data, which allowsprint processing, based on print data received from a computer (notshown) or the like, applies color conversion processing and the like tothe raster image data, and stores the processed data in a bitmap memory1406. The bitmap memory 1406 temporarily stores data to be printed, andcomprises either a page memory that stores data for one page or a bandmemory that stores data for a plurality of lines. A line buffer 1407holds line data read out from the bitmap memory 1406. Data held by theline buffer 1407 is read out by an output position correcting unit 1409to be described later. A PWM unit 1410 generates amplitude modulationdata according to the readout line data, and supplies it to the laserdriver 102 of the exposure unit 1411. As a result, the semiconductorlaser 31 of the exposure unit 1411 turns on and off according to theline data. Note that the exposure unit 1411 includes the laser driver102, semiconductor laser 31, and polygon mirror 32.

In this embodiment, by adjusting the data read start timing from theline buffer 1407 by the output position correcting unit 1409 inaccordance with the misalignment amount (θ), image distortion due to theinclination (θ) of the main scan line is reduced. This adjustmentprocessing will be described in detail below.

A misalignment correction amount arithmetic unit 1408 calculates amisalignment correction amount Δx_(n) of a line to be currently output(n-th line) based on the inclination θ stored in the misalignment amountstorage unit. The misalignment correction amount Δx_(n) is given by:

Δx _(n) =Ly(n−1)·sin θ

where Δx_(n): the misalignment correction amount of the n-th line

n: the currently scanned line number

θ: the inclination of the scan line

Ly: the distance (height of one pixel) between the scan lines

The output position correcting unit 1409 adjusts the output start timingfor each scan in accordance with the misalignment correction amountΔx_(n) calculated in this way. FIG. 37 shows the timings of videosignals output from the output position correcting unit 1409. A videosignal of the first line is output a predetermined period t₀ after thehorizontal sync signal. A video signal of the second line is outputafter a delay of Δt₂ compared to the first line. More specifically, anexposure scan starts t₀+Δt₂ after the horizontal sync signal. A delayamount Δt_(n) of a video signal of the n-th line is given by:

Δt _(n) =Δx _(n)+(dx/dt)

where Δx_(n): the misalignment correction amount of the n-th line

(dx/dt): the laser scan speed

The video signal, the timing of which is adjusted based on the aboveequation, is transmitted to the PWM unit 1410. The output from the PWMunit 1410 is sent to the exposure unit 1411 in the engine 1401, and thephotosensitive drum in the engine 1401 is exposed by a laser beam of thelaser unit, thus performing development and print processing. That is, ashift amount Δx_(n) of the write start position of each scan line isdetermined based on the misalignment amount (angle θ), and the writestart timing is delayed by Δt_(n) to attain the determined shift amount.

FIG. 36 is a view for explaining the correction state according to thisembodiment in detail. Referring to FIG. 36, the main scan line of thelaser beam has an inclination with respect to the ideal main scan line36 parallel to the rotational axis 35. The aforementioned correctioncontrol corresponds to processing for shifting the main scan startposition by Ly·sin θ for respective lines. Therefore, for a line of,e.g., n=11, the correction amount is Δx₁₁=Ly×10×sin θ.

An image output via the aforementioned process becomes an image whoseoutput positions are gradually shifted, as shown in FIG. 35C. This imagehas an inclination θ with respect to the sub-scan direction as a printdirection. A distortion generated in an image which does not undergo anycorrection, as shown in FIG. 35B, is canceled, and a high-quality imageapproximate to an ideal image shown in FIG. 35A can be obtained. Thatis, the entire image has an inclination with respect to a paper sheet,but the distortion of the image itself can be reduced.

Seventh Embodiment

The sixth embodiment has exemplified the monochrome image formingapparatus. However, the present invention can be applied to a colorimage forming apparatus. Application of the present invention to a colorimage forming apparatus which comprises independent exposure units andphotosensitive drums for respective color components will be describedbelow.

FIG. 38 is a block diagram for explaining the correction processingoperation for correcting the inclination of a scan line in a color imageforming apparatus 3800 of the seventh embodiment. The arrangement shownin FIG. 38 is obtained by developing the arrangement shown in FIG. 34for a color image forming apparatus. That is, in this arrangement, thearrangement shown in FIG. 34 is prepared in correspondence withrespective color components (cyan (C), magenta (M), yellow (Y), andblack (X) in this example). In the color image forming apparatus 3800, aplurality of color signals are generated by a printer controller 3802,and are transmitted to a printer engine 3801.

The printer engine 3801 executes print processing in practice based onimage bitmap information generated by the printer controller 3802.Reference numerals 3804C, 3804M, 3804Y, and 3804K denote horizontal syncsignal generators, which output horizontal sync signals forsynchronizing write start positions in the main scan direction to theprinter controller 3802 for respective color components to be printed.Reference numerals 3803C, 3803Y, 3803M, and 3803K denote misalignmentamount storage units, which store angles θ, each of which indicates theinclination of the main scan line 37 of the laser beam with respect tothe ideal main scan line 35, for respective color components, as shownin FIG. 33B.

In the seventh embodiment, the angle θ is stored as informationindicating the inclination of the main scan line. However, the presentinvention is not limited to this as long as the inclination of theactual main scan lien is identifiable information. As the informationstored in each of the misalignment amount storage units 3803C, 3803M,3803Y, and 3803K, the misalignment amount may be measured in themanufacturing processing of this apparatus, and may be pre-stored asinformation unique to the apparatus. Alternatively, the image formingapparatus 3800 may comprise detection mechanisms each for detecting themisalignment amount in correspondence with the photosensitive drums. Inthis case, the misalignment amounts on the respective photosensitivedrums detected by the detection mechanisms are stored in themisalignment amount storage units 3803C, 3803M, 3803Y, and 3803K. If thearrangement comprising the detection mechanisms is adopted, a change incharacteristic of the image forming apparatus (exposure units) over timecan be coped with.

The control for executing print processing by correcting the outputposition in the main scan direction to correct the misalignment amountof the main scan line stored in each of the misalignment amount storageunits 3803C, 3803M, 3803Y, and 3803K in the printer controller 3802 willbe described below.

An image generating unit 3805 generates raster image data, which allowsprint processing, based on print data received from a computer (notshown) or the like, applies color conversion processing and the like tothe raster image data, and stores the processed data in a bitmap memory3806. The bitmap memory 3806 temporarily stores data to be printed, andcomprises either a page memory that stores data for one page or a bandmemory that stores data for a plurality of lines. Line buffers 3807C,3807M, 3807Y, and 3807K hold line data read out from the bitmap memory3806 for respective color components. Data held by the line buffers3807C, 3807M, 3807Y, and 3807K are read out by output positioncorrecting units 3809C, 3809M, 3809Y, and 3809K (to be described later)for respective color components.

A monochrome determination unit 3811 determines based on data used whenthe image generating unit 3805 generates an image or based on print datasent from a computer (not shown) whether or not an image is to beprinted using only one of a plurality of colors. When the monochromedetermination unit 3811 determines that the image is to be printed usingonly one color, only a misalignment correction amount arithmetic unitcorresponding to the color to be used is activated, and misalignmentamount correction described in the sixth embodiment is executed. Thatis, one of misalignment correction amount arithmetic units 3808C, 3808M,3808Y, and 3808K corresponding to the use color acquires an inclinationθ from the corresponding one of the misalignment amount arithmetic units3808C, 3808M, 3808Y, and 3808K, and calculates a misalignment correctionamount Δx_(n). As described in the sixth embodiment, one of outputposition correcting units 3809C, 3809M, 3809Y, and 3809Y correspondingto the use color determines Δt_(n) in accordance with Δx_(n), andadjusts the output timing of a video signal (main scan start timing).

On the other hand, when an image is to be printed using a plurality ofcolors, the monochrome determination unit 3811 inhibits all themisalignment correction amount arithmetic units 3808C, 3808M, 3808Y, and3808K from executing a misalignment correction operation. Themisalignment correction amount arithmetic units 3808C, 3808M, 3808Y, and3808K whose misalignment correction operation is inhibited always outputΔx_(n)=0. Of course, the output position correcting units 3809C, 3809M,3809Y, and 3809Y may perform through operations not to apply anycorrection. The reason why the misalignment correction is skipped incase of a plurality of colors is as follows. In the arrangement whichhas the photosensitive drums for respective color components, thedirections and magnitudes of the skews or inclinations of images aredifferent for respective color components. For this reason, thecorrection amounts are different for the respective color components,and color discrepancy or the like occurs if misalignment correction isdone, thus worsening the image quality. In this case, when an image iscorrected using coordinate conversion processing or the like, the sameimage quality as in the conventional method can be obtained.

As described above, in the color image forming apparatus as well, ahigh-quality image can be obtained by the same processing as in thesixth embodiment.

Eighth Embodiment

In the seventh embodiment, the misalignment correction amount arithmeticunits and output position correcting units are prepared incorrespondence with colors. However, the misalignment correction is doneonly when the monochrome determination unit 3811 determines that animage is to be formed using only one color. That is, since themisalignment correction is always applied to one color, an arrangementwhich has a misalignment correction amount arithmetic unit and outputposition correcting unit common to all the colors may be adopted.

FIG. 39 is a block diagram showing the arrangement which has amisalignment correction amount arithmetic unit and output positioncorrecting unit common to all the color components. In this arrangement,a monochrome determination unit 911 determines whether or not amonochrome image is to be formed, and outputs information of a colorcomponent to be output (a color component to be used) to a misalignmentcorrection amount arithmetic unit 908. The misalignment correctionamount arithmetic unit 908 reads out a misalignment amount (θ) from oneof misalignment amount storage units 903C, 903M, 903Y, and 903Kcorresponding to the color component to be output, and performs acorrection arithmetic operation. An output position correcting unit 909corrects an output position in accordance with the correction arithmeticoperation result of the misalignment correction amount arithmetic unit908. In this manner, misalignment correction can be done in the samemanner as in the sixth and seventh embodiments.

According to the eighth embodiment, since the misalignment correctionamount arithmetic unit and output position correcting unit are commonlyused, cost can be reduced compared to the seventh embodiment.

Ninth Embodiment

In the sixth to eighth embodiments, the misalignment correction amountarithmetic unit calculates misalignment correction amounts for allpixels independently of a horizontal sync signal. For example, in thesixth embodiment, the output position correcting unit 1409 calculates adelay time based on the misalignment correction amount calculated by themisalignment correction amount arithmetic unit 1408. Then, each scanstart timing is determined by adding the calculated delay time to thescan start timing with reference to the horizontal sync signal.

In the ninth embodiment, the timing of the horizontal sync signal iscorrected in accordance with the arithmetic result of the misalignmentcorrection amount arithmetic unit (to shift the horizontal sync signalfor each line). FIG. 40 is a block diagram for explaining the controlarrangement for implementing correction processing that corrects theinclination of the main scan line according to the ninth embodiment.

Referring to FIG. 40, the printer engine 1401 executes actual printprocessing based on image bitmap information generated by the printercontroller 101, as in FIG. 34. In FIG. 40, a misalignment correctionsync signal generator 4008 executes the same processing as that of themisalignment correction amount arithmetic unit 1408 of the sixthembodiment. That is, the generator 4008 calculates a misalignmentcorrection amount Δx_(n) of a line to be currently output (n-th line)based on an inclination θ stored in a misalignment amount storage unit4003, and calculates a delay amount Δt_(n) of a video signal.Furthermore, the misalignment correction sync signal generator 4008generates a horizontal sync signal for the n-th line, which is delayedΔt_(n) from an actual horizontal sync signal using the calculated delayamount Δt_(n) of the video signal and the horizontal sync signal from ahorizontal sync signal generator 4004. The generated horizontal syncsignal is sent to an output data controller 4009. The output datacontroller 4009 reads out data from a line buffer 4007 in synchronismwith the horizontal sync signal for the n-th line received from themisalignment correction sync signal generator 4008, and transmits it asa video signal to a PWM unit 4010.

FIG. 41 shows the relationship between the horizontal sync signals andvideo signals according to the ninth embodiment. A video signal of eachline is output in synchronism with the horizontal sync signal for thecorresponding line output from the misalignment correction sync signalgenerator 4008. In this way, misalignment correction can be done as inthe sixth embodiment.

Tenth Embodiment

In the seventh and eighth embodiments, misalignment correction is madewhen an image is to be printed using one color. In a printer which formsa color image by a 4-pass method, misalignment correction may be madewhen an image is to be printed using a plurality of colors. FIG. 42 is ablock diagram showing the arrangement which performs misalignmentcorrection in the printer of the 4-pass method. In the arrangement shownin FIG. 42, image data of C, M, Y, and K colors saved in a bitmap memory1206 are read out onto a line buffer 1207 for each color. The writestart position of the readout data is corrected by an output positioncorrecting unit 1209, and the corrected data is output to a PWM unit1210. The PWM data output from the PWM unit 1210 is exposed anddeveloped. In the printer which has a photosensitive drum and exposureunit common to all the color components (e.g., the printer of the 4-passmethod), all colors suffer identical misalignment amounts and, hence,the inclination angles match. Therefore, the misalignment amount andmisalignment correction amount are common to all the colors. For thisreason, when the misalignment correction described in the sixthembodiment is applied to the printer of the 4-pass method, misalignmentcorrection can be done even in case of color printing.

Note that the arithmetic operations of the misalignment correctionamount and scan start timing in the sixth to ninth embodiments may beimplemented by dedicated hardware or may be implemented when a CPUexecutes a predetermined control program.

In the description of the above embodiments, θ is positive (rotation inthe counterclockwise direction). However, as can be seen from the abovedescription, the present invention can be applied to a case in which θis negative. When θ is negative, Δt_(n) also becomes negative, the timet₀ between the horizontal sync signal to each main scan write starttiming becomes shorter with increasing line number (n) in, e.g., FIG.37. In the ninth embodiment (FIG. 41) that controls the sync signaltiming itself, the interval of the horizontal sync signals changes in adirection to decrease.

According to the sixth to tenth embodiments, image distortion can beeffectively removed by a simple arrangement that shifts the write startposition of each scan line based on a misalignment amount according toan epoch-making idea that allows to print an image aslant on a printsheet and gives top priority to removal of image distortion. Morespecifically, in an image forming apparatus which forms an image byradiating a laser beam, the output pixel position in the main scandirection is shifted to cancel image distortion caused by errors of thepositional precisions and rotational axis of the apparatus without usingany complicated processing and arrangement. In this way, a high-qualityimage can be obtained with low cost. The scan start position can beobtained by an arithmetic operation, as given by Δt_(n), and can beshifted in a unit smaller than one pixel. Therefore, delicatemisalignment adjustment smaller than one pixel can be implemented.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

This application claims the benefit of Japanese Patent Application No.2005-112657 filed on Apr. 8, 2005, Japanese Patent Application No.2005-112658 filed on Apr. 8, 2005, Japanese Patent Application No.2005-118972 filed on Apr. 15, 2005, Japanese Patent Application No.2005-118973 filed on Apr. 15, 2005, Japanese Patent Application No.2005-118974 filed on Apr. 15, 2005, and Japanese Patent Application No.2005-118975 filed on Apr. 15, 2005, which are hereby incorporated byreference herein in its entirety.

1-40. (canceled)
 41. A color image forming apparatus comprising: astorage unit configured to store discrepancy amounts from an ideal scanline of a light beam modulated according to a color signal; a firstcorrecting unit configured to correct a dot to be formed according tothe light beam in a pixel unit based on a discrepancy amount obtainedfrom the storage unit; and a second correcting unit configured tofurther correct the dot in less than a pixel unit based on thediscrepancy amount by adjusting an ON/OFF ratio of dots on a neighboringscan line with respect to dots on a current scan line according to aposition in a main scan line direction.
 42. The apparatus according toclaim 41, wherein the second correcting unit is configured to correctthe dot in less than a pixel unit by dividing the scan line into aplurality of regions in the main scan line direction, and by changingthe number of ON-dots in each of the plurality of regions.
 43. Theapparatus according to claim 41, wherein the apparatus is configured toform an image by using CMYK toners, and the storage unit is configuredto store the discrepancy amounts in respect to each of CMYK.
 44. Amethod of controlling a color image forming apparatus, the methodcomprising: a first correcting step of correcting a dot to be formedaccording to the light beam in a pixel unit based on a discrepancyamount obtained from a storage unit which stores discrepancy amountsfrom an ideal scan line of a light beam modulated according to a colorsignal; and a second correcting step of further correcting the dot inless than a pixel unit based on the discrepancy amount by adjusting anON/OFF ratio of dots on a neighboring scan line with respect to dots ona current scan line according to a position in a main scan linedirection.
 45. The method according to claim 44, wherein the secondcorrecting step corrects the dot in less than a pixel unit by dividingthe scan line into a plurality of regions in the main scan linedirection, and by changing the number of ON-dots in each of theplurality of regions.
 46. The method according to claim 44, wherein thecolor image forming apparatus is configured to form an image by usingCMYK toners, and the storage unit is configured to store the discrepancyamounts in respect to each of CMYK.